diff --git a/projects/packages/forms/changelog/add-forms-submit-timer b/projects/packages/forms/changelog/add-forms-submit-timer
new file mode 100644
index 000000000000..4c0878bfe7b1
--- /dev/null
+++ b/projects/packages/forms/changelog/add-forms-submit-timer
@@ -0,0 +1,4 @@
+Significance: minor
+Type: added
+
+Add form fill duration to form entries.
diff --git a/projects/packages/forms/src/contact-form/class-contact-form-endpoint.php b/projects/packages/forms/src/contact-form/class-contact-form-endpoint.php
index 8c1d7f59cf47..8bc2a2ec8c22 100644
--- a/projects/packages/forms/src/contact-form/class-contact-form-endpoint.php
+++ b/projects/packages/forms/src/contact-form/class-contact-form-endpoint.php
@@ -659,6 +659,15 @@ public function get_item_schema() {
 			'readonly'    => true,
 		);
 
+		$schema['properties']['form_fill_duration'] = array(
+			'description' => __( 'The duration in seconds from first user interaction to form submission. Null when the duration is unknown, such as for submissions predating this feature.', 'jetpack-forms' ),
+			'type'        => array( 'integer', 'null' ),
+			'context'     => array( 'view', 'edit', 'embed' ),
+			// No sanitize_callback: the field is readonly and sanitized on storage, and
+			// `absint` would coerce a legitimate null into 0.
+			'readonly'    => true,
+		);
+
 		$schema['properties']['browser'] = array(
 			'description' => __( 'The browser and platform used to submit the form.', 'jetpack-forms' ),
 			'type'        => 'string',
@@ -991,6 +1000,10 @@ public function prepare_item_for_response( $item, $request ) {
 			$data['country_code'] = $feedback_response->get_country_code();
 		}
 
+		if ( rest_is_field_included( 'form_fill_duration', $fields ) ) {
+			$data['form_fill_duration'] = $feedback_response->get_form_fill_duration();
+		}
+
 		if ( rest_is_field_included( 'browser', $fields ) ) {
 			$data['browser'] = $feedback_response->get_browser();
 		}
diff --git a/projects/packages/forms/src/contact-form/class-contact-form.php b/projects/packages/forms/src/contact-form/class-contact-form.php
index 9ab52526846c..1754c22c199d 100644
--- a/projects/packages/forms/src/contact-form/class-contact-form.php
+++ b/projects/packages/forms/src/contact-form/class-contact-form.php
@@ -1544,6 +1544,7 @@ public static function parse( $attributes, $content, $context = array() ) {
 			id='contact-form-$id'
 			class='{$container_classes_string}'
 			data-wp-interactive='jetpack/form' " . wp_interactivity_data_wp_context( $context ) . "
+			data-wp-on--focusin=\"actions.trackFirstInteraction\"
 			data-wp-watch--scroll-to-wrapper=\"callbacks.scrollToWrapper\"
 		>\n";
 
@@ -1632,6 +1633,10 @@ class='" . esc_attr( $form_classes ) . "' $form_aria_label
 				$r .= '<input type="submit" style="display: none;" />';
 			}
 			$r .= "<input type='hidden' name='jetpack_contact_form_jwt' value='" . esc_attr( $form->get_jwt() ) . "' />\n";
+			// Left empty on purpose: the view script fills this in on submit. An empty
+			// value is stored as null so "never interacted with" stays distinguishable
+			// from "filled out in under a second".
+			$r .= "<input type='hidden' name='" . esc_attr( Feedback::FORM_FILL_DURATION_FIELD ) . "' value='' />\n";
 			$r .= $form->body;
 
 			if ( $is_multistep ) {
diff --git a/projects/packages/forms/src/contact-form/class-feedback.php b/projects/packages/forms/src/contact-form/class-feedback.php
index c4bcbaaf281e..c5d30b2a2876 100644
--- a/projects/packages/forms/src/contact-form/class-feedback.php
+++ b/projects/packages/forms/src/contact-form/class-feedback.php
@@ -52,6 +52,19 @@ class Feedback {
 	 */
 	public const IS_TEST_META_KEY = '_feedback_is_test';
 
+	/**
+	 * Name of the hidden POST field carrying the form fill duration.
+	 *
+	 * Prefixed because submitted fields share one flat POST namespace with author-defined
+	 * fields, whose names a site owner can set by hand. An unprefixed `form_fill_duration`
+	 * field would silently overwrite this one.
+	 *
+	 * @since $$next-version$$
+	 *
+	 * @var string
+	 */
+	public const FORM_FILL_DURATION_FIELD = 'jetpack_form_fill_duration';
+
 	/**
 	 * Cache key for the source post IDs list.
 	 *
@@ -248,6 +261,15 @@ public static function maybe_backfill_source_meta( $post_id, $feedback ) {
 	 */
 	protected $country_code = null;
 
+	/**
+	 * The form fill duration in seconds.
+	 *
+	 * Tracks how long the user spent filling out the form (from first interaction to submission).
+	 *
+	 * @var int|null
+	 */
+	protected $form_fill_duration = null;
+
 	/**
 	 * The subject of the feedback entry.
 	 *
@@ -427,10 +449,11 @@ private function load_from_post( WP_Post $feedback_post ) {
 			! empty( $parsed_content['is_test'] )
 		);
 
-		$this->ip_address   = $parsed_content['ip'] ?? $this->get_first_field_of_type( 'ip' );
-		$this->country_code = $parsed_content['country_code'] ?? null;
-		$this->user_agent   = $parsed_content['user_agent'] ?? null;
-		$this->subject      = $parsed_content['subject'] ?? $this->get_first_field_of_type( 'subject' );
+		$this->ip_address         = $parsed_content['ip'] ?? $this->get_first_field_of_type( 'ip' );
+		$this->country_code       = $parsed_content['country_code'] ?? null;
+		$this->user_agent         = $parsed_content['user_agent'] ?? null;
+		$this->form_fill_duration = $parsed_content['form_fill_duration'] ?? null;
+		$this->subject            = $parsed_content['subject'] ?? $this->get_first_field_of_type( 'subject' );
 
 		$this->notification_recipients = $parsed_content['notification_recipients'] ?? array();
 		$this->logged_in_user          = $parsed_content['logged_in_user'] ?? null;
@@ -493,14 +516,15 @@ private function load_from_submission( $post_data, $form, $current_post = null,
 		$this->form_id     = $form_id_attribute > 0 ? $form_id_attribute : null;
 
 		// If post_data is provided, use it to populate fields.
-		$this->fields          = $this->get_computed_fields( $post_data, $form );
-		$this->ip_address      = Contact_Form_Plugin::get_ip_address();
-		$this->country_code    = $this->get_country_code_from_ip( $this->ip_address );
-		$this->user_agent      = isset( $_SERVER['HTTP_USER_AGENT'] ) ? filter_var( wp_unslash( $_SERVER['HTTP_USER_AGENT'] ) ) : null;
-		$this->subject         = $this->get_computed_subject( $post_data, $form );
-		$this->author_data     = Feedback_Author::from_submission( $post_data, $form );
-		$this->comment_content = $this->get_computed_comment_content( $post_data, $form );
-		$this->has_consent     = $this->get_computed_consent( $post_data, $form );
+		$this->fields             = $this->get_computed_fields( $post_data, $form );
+		$this->ip_address         = Contact_Form_Plugin::get_ip_address();
+		$this->country_code       = $this->get_country_code_from_ip( $this->ip_address );
+		$this->user_agent         = isset( $_SERVER['HTTP_USER_AGENT'] ) ? filter_var( wp_unslash( $_SERVER['HTTP_USER_AGENT'] ) ) : null;
+		$this->form_fill_duration = $this->get_computed_form_fill_duration( $post_data );
+		$this->subject            = $this->get_computed_subject( $post_data, $form );
+		$this->author_data        = Feedback_Author::from_submission( $post_data, $form );
+		$this->comment_content    = $this->get_computed_comment_content( $post_data, $form );
+		$this->has_consent        = $this->get_computed_consent( $post_data, $form );
 
 		$this->notification_recipients = $this->get_computed_notification_recipients( $post_data, $form );
 
@@ -1124,6 +1148,17 @@ public function get_country_code() {
 		return $this->country_code;
 	}
 
+	/**
+	 * Get the form fill duration in seconds.
+	 *
+	 * Represents the time from first user interaction to form submission.
+	 *
+	 * @return int|null
+	 */
+	public function get_form_fill_duration() {
+		return $this->form_fill_duration;
+	}
+
 	/**
 	 * Get the emoji flag for the country.
 	 *
@@ -1567,6 +1602,7 @@ public function serialize() {
 				'ip'                      => $this->ip_address,
 				'country_code'            => $this->country_code,
 				'user_agent'              => $this->user_agent,
+				'form_fill_duration'      => $this->form_fill_duration,
 				'notification_recipients' => $this->notification_recipients,
 				'logged_in_user'          => $this->logged_in_user,
 			),
@@ -2254,6 +2290,40 @@ private function get_computed_consent( $post_data, $form ) {
 		return false;
 	}
 
+	/**
+	 * Gets the computed form fill duration, in seconds.
+	 *
+	 * The value is supplied by the view script as a hidden field, so it is submitter-controlled
+	 * and cannot be trusted. Anything that is not a plain sequence of digits is treated as
+	 * unknown and stored as null, rather than being coerced into a number that would read as a
+	 * real measurement: `absint()` alone would turn "abc" into 0 (indistinguishable from a
+	 * genuine sub-second fill), "-1" into 1, and a value past PHP_INT_MAX into a float, which
+	 * would contradict the integer type the REST schema advertises.
+	 *
+	 * The value is left empty when the submitter never interacted with the form, or ran without
+	 * JavaScript, which is also an unknown duration.
+	 *
+	 * @since $$next-version$$
+	 *
+	 * @param array $post_data The post data from the form submission.
+	 * @return int|null
+	 */
+	private function get_computed_form_fill_duration( $post_data ) {
+		if ( ! isset( $post_data[ self::FORM_FILL_DURATION_FIELD ] ) ) {
+			return null;
+		}
+
+		$raw = $post_data[ self::FORM_FILL_DURATION_FIELD ];
+
+		// is_scalar() has to come first so an array-shaped POST does not blow up on the cast.
+		if ( ! is_scalar( $raw ) || ! ctype_digit( (string) $raw ) ) {
+			return null;
+		}
+
+		// Clamp so an abandoned tab left open for days cannot skew aggregates.
+		return min( (int) $raw, DAY_IN_SECONDS );
+	}
+
 	/**
 	 * Gets the computed notification recipients.
 	 *
diff --git a/projects/packages/forms/src/modules/file-field/view.js b/projects/packages/forms/src/modules/file-field/view.js
index a0307367a22d..6632f7c76907 100644
--- a/projects/packages/forms/src/modules/file-field/view.js
+++ b/projects/packages/forms/src/modules/file-field/view.js
@@ -304,6 +304,10 @@ const { state, actions } = store( NAMESPACE, {
 		 */
 		fileDropped: event => {
 			event.preventDefault();
+			// A drop fires no focus event, so the form's `focusin` handler never sees it.
+			// Without this, dropping a file and submitting would report the fill as starting
+			// at the submit button rather than at the drop.
+			jetpackFormStore.actions.trackFirstInteraction();
 			if ( event.dataTransfer ) {
 				for ( const item of Array.from( event.dataTransfer.items ) ) {
 					if ( item.webkitGetAsEntry()?.isDirectory ) {
diff --git a/projects/packages/forms/src/modules/form/shared.ts b/projects/packages/forms/src/modules/form/shared.ts
index d93204cf6858..88e17936ef9f 100644
--- a/projects/packages/forms/src/modules/form/shared.ts
+++ b/projects/packages/forms/src/modules/form/shared.ts
@@ -9,7 +9,7 @@ const debug = debugFactory( 'jetpack-forms:interactivity' );
 const NAMESPACE = 'jetpack/form';
 const config = getConfig( NAMESPACE );
 
-const getForm = ( formHash: string ) => {
+export const getForm = ( formHash: string ) => {
 	return document.getElementById( 'jp-form-' + formHash ) as HTMLFormElement | null;
 };
 
diff --git a/projects/packages/forms/src/modules/form/view.js b/projects/packages/forms/src/modules/form/view.js
index 8a5aa3b359f7..c1594c09015a 100644
--- a/projects/packages/forms/src/modules/form/view.js
+++ b/projects/packages/forms/src/modules/form/view.js
@@ -14,7 +14,7 @@ import {
 import { validateField, isEmptyValue } from '../../contact-form/js/validate-helper.js';
 import { getRating } from '../field-rating/view.js';
 import { maybeAddColonToLabel, maybeTransformValue, getImages, getUrl } from './helpers.js';
-import { focusNextInput, submitForm } from './shared.ts';
+import { focusNextInput, getForm, submitForm } from './shared.ts';
 // Import field type icons view to register its callbacks.
 import './field-type-icons-view.js';
 
@@ -28,6 +28,9 @@ const NAMESPACE = 'jetpack/form';
 const config = getConfig( NAMESPACE );
 let errorTimeout = null;
 
+// Must match Feedback::FORM_FILL_DURATION_FIELD in src/contact-form/class-feedback.php.
+const FORM_FILL_DURATION_FIELD = 'jetpack_form_fill_duration';
+
 const updateField = ( fieldId, value, showFieldError = false, validatorCallback = null ) => {
 	const context = getContext();
 	let field = context.fields[ fieldId ];
@@ -608,10 +611,33 @@ const { state, actions } = store( NAMESPACE, {
 			actions.updateField( context.fieldId, event.target.value, true );
 		},
 
+		/**
+		 * Start the fill timer on the submitter's first interaction with the form.
+		 *
+		 * Bound to `focusin` on the form wrapper, which covers every focusable control. File
+		 * drag-and-drop fires no focus event, so `jetpack/field-file` calls this directly.
+		 *
+		 * Uses `performance.now()` rather than `Date.now()`: it is monotonic, so a wall-clock
+		 * step backward mid-fill cannot produce a negative duration. Compared against null
+		 * rather than tested for truthiness, since a legitimate reading can be 0.
+		 */
+		trackFirstInteraction: () => {
+			const context = getContext();
+
+			if ( context.formFirstInteractionTime == null ) {
+				context.formFirstInteractionTime = performance.now();
+			}
+		},
+
 		onFormReset: () => {
 			const context = getContext();
 			context.fields = [];
 			context.showErrors = false;
+			// Start the fill timer over. Without this, going back from the success panel and
+			// filling the form in again would report a duration that also covers the first
+			// submission and the time spent reading the confirmation. The hidden input needs no
+			// clearing here because the write on submit is unconditional.
+			context.formFirstInteractionTime = null;
 
 			// Dispatch custom events to reset all fields
 			const formElement = document.getElementById( context.elementId );
@@ -664,6 +690,24 @@ const { state, actions } = store( NAMESPACE, {
 
 			context.isSubmitting = true;
 
+			// Record the fill duration in the DOM before submitting. This has to happen outside
+			// the `useAjax` branch below: non-AJAX forms submit natively, so the value is only
+			// sent if it is already on the hidden input by this point.
+			//
+			// The write is unconditional so the input can never carry a value left over from an
+			// earlier fill — an unknown duration explicitly writes an empty string, which the
+			// server stores as null.
+			const durationField = getForm( context.formHash )?.querySelector(
+				`input[name="${ FORM_FILL_DURATION_FIELD }"]`
+			);
+
+			if ( durationField ) {
+				durationField.value =
+					context.formFirstInteractionTime == null
+						? ''
+						: Math.round( ( performance.now() - context.formFirstInteractionTime ) / 1000 ); // Duration in seconds.
+			}
+
 			if ( context.useAjax ) {
 				event.preventDefault();
 				event.stopPropagation();
diff --git a/projects/packages/forms/tests/php/contact-form/Contact_Form_Endpoint_Test.php b/projects/packages/forms/tests/php/contact-form/Contact_Form_Endpoint_Test.php
index 3802e737b2e7..1c288a0c24b8 100644
--- a/projects/packages/forms/tests/php/contact-form/Contact_Form_Endpoint_Test.php
+++ b/projects/packages/forms/tests/php/contact-form/Contact_Form_Endpoint_Test.php
@@ -179,6 +179,11 @@ public function test_item_schema() {
 		$this->assertArrayHasKey( 'is_test', $schema_properties );
 		$this->assertEquals( 'boolean', $schema_properties['is_test']['type'] );
 		$this->assertArrayHasKey( 'preview_url', $schema_properties );
+		$this->assertArrayHasKey( 'form_fill_duration', $schema_properties );
+
+		// The duration is null whenever it is unknown, so the schema has to allow both.
+		$this->assertContains( 'integer', $schema_properties['form_fill_duration']['type'] );
+		$this->assertContains( 'null', $schema_properties['form_fill_duration']['type'] );
 
 		// Verify logged_in_user schema structure
 		$logged_in_user_schema = $schema_properties['logged_in_user'];
diff --git a/projects/packages/forms/tests/php/contact-form/Contact_Form_Test.php b/projects/packages/forms/tests/php/contact-form/Contact_Form_Test.php
index 735a9368031a..65e400160147 100644
--- a/projects/packages/forms/tests/php/contact-form/Contact_Form_Test.php
+++ b/projects/packages/forms/tests/php/contact-form/Contact_Form_Test.php
@@ -1306,6 +1306,29 @@ public function test_token_with_curly_brackets_can_be_replaced() {
 		$this->assertEquals( 'Chicago', $plugin->replace_tokens_with_input( $subject, $field_values ) );
 	}
 
+	/**
+	 * The rendered form must carry the hidden duration input and the focusin binding that
+	 * populates it. The storage-level tests build their POST data by hand, so without this
+	 * the whole feature could be removed from the markup and they would all still pass.
+	 */
+	public function test_rendered_form_carries_form_fill_duration_markup() {
+		$html = do_shortcode( "[contact-form][contact-field label='Name' type='name' required='1'/][/contact-form]" );
+
+		// Asserted as a pair: an unanchored `value=''` would also match the Name field, which
+		// renders an empty value of its own, so it would pass even if the duration input
+		// carried a default.
+		$this->assertStringContainsString(
+			"name='" . Feedback::FORM_FILL_DURATION_FIELD . "' value=''",
+			$html,
+			'The duration input should render empty so an unrecorded duration stores as null'
+		);
+		$this->assertStringContainsString(
+			'data-wp-on--focusin="actions.trackFirstInteraction"',
+			$html,
+			'The form wrapper should bind focusin to start the fill timer'
+		);
+	}
+
 	/**
 	 * Tests that the field attributes remain the same when no escaping is necessary.
 	 *
diff --git a/projects/packages/forms/tests/php/contact-form/Feedback_Author_Metadata_Test.php b/projects/packages/forms/tests/php/contact-form/Feedback_Author_Metadata_Test.php
index 9c2d8f97e6c5..9194836dac85 100644
--- a/projects/packages/forms/tests/php/contact-form/Feedback_Author_Metadata_Test.php
+++ b/projects/packages/forms/tests/php/contact-form/Feedback_Author_Metadata_Test.php
@@ -12,6 +12,7 @@
 require_once __DIR__ . '/class-utility.php';
 
 use PHPUnit\Framework\Attributes\CoversClass;
+use PHPUnit\Framework\Attributes\DataProvider;
 use WorDBless\BaseTestCase;
 
 /**
@@ -383,4 +384,218 @@ public function test_author_first_last_on_submission() {
 		$this->assertSame( 'Jane', $response->get_author_first_name(), 'First name getter should return raw first name' );
 		$this->assertSame( 'Doe', $response->get_author_last_name(), 'Last name getter should return raw last name' );
 	}
+
+	/**
+	 * Test that form_fill_duration is included in serialized response and persists after save/load.
+	 */
+	public function test_form_fill_duration_persists_after_save() {
+
+		$form_id = Utility::get_form_id();
+		// Create a form submission with form_fill_duration
+		$_post_data = Utility::get_post_request(
+			array(
+				'name'                             => 'John Doe',
+				'email'                            => 'john@example.com',
+				'message'                          => 'Test message',
+				Feedback::FORM_FILL_DURATION_FIELD => '45', // 45 seconds
+			),
+			'g' . $form_id
+		);
+
+		$form = new Contact_Form(
+			array(
+				'title'       => 'Test Form',
+				'description' => 'This is a test form.',
+			),
+			"[contact-field label='Name' type='name' required='1'/][contact-field label='Email' type='email' required='1'/][contact-field label='Message' type='textarea' required='1'/]"
+		);
+
+		// Create a contact form response
+		$response         = Feedback::from_submission( $_post_data, $form );
+		$feedback_post_id = $response->save();
+		$saved_response   = Feedback::get( $feedback_post_id );
+
+		// The form_fill_duration should be present and match the test value.
+		$this->assertNotEmpty( $response->get_form_fill_duration(), 'Form fill duration should not be empty' );
+		$this->assertNotEmpty( $saved_response->get_form_fill_duration(), 'Form fill duration should not be empty after save/load' );
+		$this->assertEquals( $response->get_form_fill_duration(), $saved_response->get_form_fill_duration(), 'Form fill duration should match after save/load' );
+		$this->assertEquals( 45, $saved_response->get_form_fill_duration(), 'Form fill duration should be 45 seconds' );
+		$this->assertIsInt( $saved_response->get_form_fill_duration(), 'Form fill duration should be an integer' );
+	}
+
+	/**
+	 * An empty form_fill_duration means the duration is unknown, and must be stored as null
+	 * rather than as 0 so it stays distinct from a form that really was filled out instantly.
+	 *
+	 * @dataProvider provide_unknown_form_fill_durations
+	 *
+	 * @param array  $extra_post_data Submission data to merge in.
+	 * @param string $message         Assertion message.
+	 */
+	#[DataProvider( 'provide_unknown_form_fill_durations' )]
+	public function test_form_fill_duration_is_null_when_unknown( $extra_post_data, $message ) {
+		$form_id = Utility::get_form_id();
+
+		$_post_data = Utility::get_post_request(
+			array_merge(
+				array(
+					'name'    => 'John Doe',
+					'email'   => 'john@example.com',
+					'message' => 'Test message',
+				),
+				$extra_post_data
+			),
+			'g' . $form_id
+		);
+
+		$form = new Contact_Form(
+			array(
+				'title'       => 'Test Form',
+				'description' => 'This is a test form.',
+			),
+			"[contact-field label='Name' type='name' required='1'/][contact-field label='Email' type='email' required='1'/][contact-field label='Message' type='textarea' required='1'/]"
+		);
+
+		$response       = Feedback::from_submission( $_post_data, $form );
+		$saved_response = Feedback::get( $response->save() );
+
+		$this->assertNull( $response->get_form_fill_duration(), $message );
+		$this->assertNull( $saved_response->get_form_fill_duration(), $message . ' (after save/load)' );
+	}
+
+	/**
+	 * Data provider for test_form_fill_duration_is_null_when_unknown.
+	 *
+	 * @return array
+	 */
+	public static function provide_unknown_form_fill_durations() {
+		return array(
+			'empty value (no interaction recorded)' => array(
+				array( Feedback::FORM_FILL_DURATION_FIELD => '' ),
+				'An empty duration should be stored as null, not 0',
+			),
+			'field absent entirely'                 => array(
+				array(),
+				'A missing duration should be stored as null',
+			),
+			'non-numeric text'                      => array(
+				array( Feedback::FORM_FILL_DURATION_FIELD => 'abc' ),
+				'Non-numeric input should be null, not absint()-ed to 0',
+			),
+			'whitespace only'                       => array(
+				array( Feedback::FORM_FILL_DURATION_FIELD => ' ' ),
+				'Whitespace should be null, not 0',
+			),
+			'negative value'                        => array(
+				array( Feedback::FORM_FILL_DURATION_FIELD => '-1' ),
+				'A negative duration is not meaningful and should be null, not flipped positive',
+			),
+			'scientific notation'                   => array(
+				array( Feedback::FORM_FILL_DURATION_FIELD => '1e10' ),
+				'Scientific notation should be null rather than expanded to 10000000000',
+			),
+			'decimal'                               => array(
+				array( Feedback::FORM_FILL_DURATION_FIELD => '1.5' ),
+				'A non-integer duration should be null',
+			),
+			'array-shaped POST'                     => array(
+				array( Feedback::FORM_FILL_DURATION_FIELD => array( '1' ) ),
+				'An array-shaped value should be null and must not fatal on the cast',
+			),
+		);
+	}
+
+	/**
+	 * Entries created before this feature existed have no duration recorded at all. This is the
+	 * case the null design exists for, and it goes through load_from_post() rather than
+	 * from_submission().
+	 */
+	public function test_form_fill_duration_is_null_for_legacy_entries() {
+		$feedback_id = Utility::create_legacy_feedback();
+		$response    = Feedback::get( $feedback_id );
+
+		$this->assertNull( $response->get_form_fill_duration(), 'An entry predating this feature should report a null duration' );
+	}
+
+	/**
+	 * A value past PHP_INT_MAX must not become a float — the REST schema advertises
+	 * array( 'integer', 'null' ), and a float would JSON-encode as 9.223372036854776e+18.
+	 */
+	public function test_form_fill_duration_rejects_overflow() {
+		$saved_response = $this->save_submission_with_duration( '-9223372036854775808' );
+
+		$this->assertNull( $saved_response->get_form_fill_duration(), 'An overflowing value should be null rather than a float' );
+	}
+
+	/**
+	 * An implausibly long duration is clamped so a tab left open for days cannot skew aggregates.
+	 */
+	public function test_form_fill_duration_is_clamped() {
+		$saved_response = $this->save_submission_with_duration( (string) ( DAY_IN_SECONDS * 30 ) );
+
+		$this->assertSame( DAY_IN_SECONDS, $saved_response->get_form_fill_duration(), 'A duration beyond a day should be clamped to DAY_IN_SECONDS' );
+		$this->assertIsInt( $saved_response->get_form_fill_duration(), 'The clamped duration should still be an integer' );
+	}
+
+	/**
+	 * Saves a submission carrying the given raw duration value and returns the reloaded entry.
+	 *
+	 * @param mixed $raw_duration The raw POST value for the duration field.
+	 * @return Feedback
+	 */
+	private function save_submission_with_duration( $raw_duration ) {
+		$form_id = Utility::get_form_id();
+
+		$_post_data = Utility::get_post_request(
+			array(
+				'name'                             => 'John Doe',
+				'email'                            => 'john@example.com',
+				'message'                          => 'Test message',
+				Feedback::FORM_FILL_DURATION_FIELD => $raw_duration,
+			),
+			'g' . $form_id
+		);
+
+		$form = new Contact_Form(
+			array(
+				'title'       => 'Test Form',
+				'description' => 'This is a test form.',
+			),
+			"[contact-field label='Name' type='name' required='1'/][contact-field label='Email' type='email' required='1'/][contact-field label='Message' type='textarea' required='1'/]"
+		);
+
+		$response = Feedback::from_submission( $_post_data, $form );
+
+		return Feedback::get( $response->save() );
+	}
+
+	/**
+	 * A real, sub-second fill still records 0 — that is a known duration, not an unknown one.
+	 */
+	public function test_form_fill_duration_zero_is_preserved() {
+		$form_id = Utility::get_form_id();
+
+		$_post_data = Utility::get_post_request(
+			array(
+				'name'                             => 'John Doe',
+				'email'                            => 'john@example.com',
+				'message'                          => 'Test message',
+				Feedback::FORM_FILL_DURATION_FIELD => '0',
+			),
+			'g' . $form_id
+		);
+
+		$form = new Contact_Form(
+			array(
+				'title'       => 'Test Form',
+				'description' => 'This is a test form.',
+			),
+			"[contact-field label='Name' type='name' required='1'/][contact-field label='Email' type='email' required='1'/][contact-field label='Message' type='textarea' required='1'/]"
+		);
+
+		$response       = Feedback::from_submission( $_post_data, $form );
+		$saved_response = Feedback::get( $response->save() );
+
+		$this->assertSame( 0, $saved_response->get_form_fill_duration(), 'An explicit 0 duration should be preserved as 0, not converted to null' );
+	}
 }
diff --git a/projects/packages/forms/tests/php/contact-form/class-utility.php b/projects/packages/forms/tests/php/contact-form/class-utility.php
index d5640d7d8a65..0d6404abac00 100644
--- a/projects/packages/forms/tests/php/contact-form/class-utility.php
+++ b/projects/packages/forms/tests/php/contact-form/class-utility.php
@@ -175,7 +175,7 @@ public static function get_post_request( $values, $form_id = null, $post_id = 0
 		$prefix    = $form_id ? $form_id : 'g' . $post_id;
 		$post_data = array();
 		foreach ( $values as $key => $val ) {
-			if ( strpos( $key, 'contact-form' ) === 0 || strpos( $key, 'action' ) === 0 ) {
+			if ( strpos( $key, 'contact-form' ) === 0 || strpos( $key, 'action' ) === 0 || $key === Feedback::FORM_FILL_DURATION_FIELD ) {
 				$post_data[ $key ] = $val;
 			} else {
 				$post_data[ $prefix . '-' . $key ] = $val;
diff --git a/projects/plugins/jetpack/changelog/add-forms-submit-timer b/projects/plugins/jetpack/changelog/add-forms-submit-timer
new file mode 100644
index 000000000000..67502098dd6c
--- /dev/null
+++ b/projects/plugins/jetpack/changelog/add-forms-submit-timer
@@ -0,0 +1,4 @@
+Significance: minor
+Type: enhancement
+
+Forms: Add form fill duration to feedback entries.