Filter the video array of the Video SEO plugin to include duration from a custom field

video-filter.php

<?php

/**
 * Filter the vid array to include duration. 
 *
 * @param array $vid The video array as determined by the indexation process
 *
 * @return array
 */
function filter_jwplayer_duration( $vid ) {
	if ( !isset( $vid['post_id'] ) )
		return $vid;

	$duration = get_post_meta( $vid['post_id'], 'duration', true );
	if ( !$duration )
		$duration = get_post_meta( $vid['post_id'], 'Duration', true );
	
	if ( $duration && !empty( $duration ) )
		$vid['duration'] = $duration;
		
	return $vid;
}
add_filter( 'wpseo_video_jwplayer_details', 'filter_jwplayer_duration' );