Wishes is probably about 15 to 20 minutes. If Wishes is at 8:00 and you watch it from the beach at the Poly you would probably get up to 'Ohanas by 8:30. As they ask you to arrive for your reservations 10 to 15 minutes early you should be fine for booking your dinner at 8:45 or later.
Another option that we always do is to watch Wishes from 'Ohana. For 8:00 Wishes I would get at least 6:30 or 6:45 reservations. When you check in ask for a window seat in the main dining room. Not all window seats have a view of the castle so try to get a front window. By the time you are enjoying desert they dim the lights and pipe in the music for Wishes and the show will begin. If this is too early for dinner and you watch Wishes from the beach at the Poly make sure you get a spot near a speaker so you can hear the naration - it really adds to the enjoyment of the show.
Linda