Customize when to update feed

Yes, but not with the plugin.
PLease write a custom plugin or copy the code to your functions.php of the theme.

add_filter( 'wp_feed_cache_transient_lifetime', create_function( '$a', 'return 1800;' ) );

1800 in seconds

more background in my post: http://wpengineer.com/1820/feed-cache-in-wordpress/