Using c.bavota and from the WordPress Codex for human_time_diff() these function return a readable time since last post for custom post types

Time since last custom post type

/**
 * Display time since post was published
 *
 * @uses	human_time_diff()  Return time difference in easy to read format
 * @uses	get_the_time()  Get the time the post was published
 * @uses	current_time()  Get the current time
 *
 * @return	string  Timestamp since post was published
 *
 * @author c.bavota
 */
function get_time_since_posted( ) {

	$time_since_posted = human_time_diff( get_the_time( 'U' ), current_time( 'timestamp' ) ) . ' ago';

	return $time_since_posted;

}

function jw_time_since_last_post($post_type)
{

	$args = array(
		'post_type' 		=> $post_type,
		'posts_per_page' 	=> 1,
		'no_found_rows' 	=> true,
		'fields' 		=> 'ids'

	);

	// The Query
	$the_query = new WP_Query( $args );

	// The Loop
	if ( $the_query->have_posts() ) {

		while ( $the_query->have_posts() ) {
			$the_query->the_post();

			return get_time_since_posted( );
		}

	} else {
		// no posts found
	}
	/* Restore original Post Data */
	wp_reset_postdata();
		
}