As far as I know mormally the latest time is 6pm but I have heard that they are allowing ceremonies later than that.
Please remember that you are dealing with Disney and their policy is usually if you don't ask you don't get - they're not the best at volunteering information. So call your SM and ask what is the latest you can have a ceremony - I've certainly seen a couple of nighttime weddings at the Yacht Club Gazebo but I'm unsure about the WP.