// Add a new setting in wp-admin > Settings > General

add_action( 'admin_init', function() {

   register_setting( 'general', 'custom_heartbeat_interval', 'intval' );

   add_settings_field( 'custom_heartbeat_interval', 'Heartbeat Interval', function() {

       $interval = get_option( 'custom_heartbeat_interval', 120 );

       echo "<input type='number' name='custom_heartbeat_interval' value='".absint($interval)."' min='15' max='120' /> seconds";

   }, 'general' );

});

add_filter( 'heartbeat_settings', function( $settings ) {

   $settings['interval'] = get_option( 'custom_heartbeat_interval', 120 );

   return $settings;

});