I would say one hour before the actual start of Wishes. However, know that not all tables at O'Hana have a view of Wishes. One option to guarantee you see them is to make a reservation more like two hours before then go see them from the beach at the Poly.
If you want to watch them from the restaurant, I'd say an hour since you often have to wait to be seated if you arrive just 'on time'. If you can't get that, then arrive early and put your name in. If that doesn't work out, you can always watch from outside the poly or take the monorail back and watch from main st.