Any time! If you keep your receipt from your meal the CM's at CG will allow you back up to the observation deck to watch Wishes. If you want to be at your table I would suggest planning an ADR 30-45 minutes prior to the show to ensure that you are seated. My DW and I had an ADR 15 minutes before and were seated on time but we still ended up walking out to the deck for Wishes rather than sitting at our table.
I'd even go for an hour to an hour and a half before Wishes if you want a shot at watching it from your table (assuming you get a table by a window with a MK view)
If you go the the Disney website. You will find the park hours calendar under Parks-->Magic Kingdom-->Calendar. Go to the month you need and click on the hours for the day you are planning. This will bring up a schedule for the day including the parades and Wishes. While there are exceptions, generally speaking Wishes is normally scheduled for the same time that MK closes, however, last August there was at least one weekend night when Wishes was at 8 or 9 and the park was open until 11.