How to change Akismet interval to three days instead of 15 days for deleting spam comments.

Add to your theme’s functions.php or site-specific plugin file:

add_filter( 'akismet_delete_comment_interval', 'custom_set_delete_interval' );
function custom_set_delete_interval() {
        return 3;
}

This changes Akismet’s interval to 3 days, after which comments held as spam are deleted.