It largely depends on where you are going to eat and how fast you tend to eat. Do you order an appetizer, entree, and dessert? If you order all three, I'd allocate about 90 minutes to two hours for dinner.
However, if you're planning on eating at Biergarten (as indicated in your other post), if you get a 6:30 pm ADR, that should give you plenty of time to eat and then find a place to watch IllumiNations. I would guess that you could go as late as 7 and still have enough time, but better to have to much than not enough.
Unlike some shows, IllumiNations can be viewed from almost all around World Showcase Lagoon. Just make sure you can see the center of the lagoon from where you're standing and you should be fine.