That depends on a few factors:
1) Are you the type to enjoy a drink or two before dinner?
2) Do you enjoy having appetizers, main & dessert?
3) Do you like to linger over your meal or get in and get out?
4) How many are in your party?
You will be visiting dining during a very crowded and busy time at WDW. There is a "pre-show" at La Nouba that will start at 8:30ish. It's not a "don't miss" but it will keep you plenty entertained if you should happen to get to the theater earlier than expected.
If I am just enjoying a main course, I allow 1 hour for dinner. If I want to add courses or want to enjoy the atmosphere, about 90 minutes. Assuming that no matter what time you make the reservation for, you are not likely to get in *exactly* at that time. . . .
I would recommend between 6:30 - 7:00 as your reservation time. If you get in early and finish eating early, you can explore Disney's West Side and the La Nouba gift shop. If you end up with slow service, you won't feel rushed to get to the show.