It depends on how many courses you order and how busy is the restaurant. I would typically allow about 90 minutes for a TS. We usually order an appetizer, entree, and dessert.
In terms of IllumiNations, I think you'll be cutting it close. Assuming IllumiNations is at 9 pm, that would give you 65 minutes to be seated, eat, and pay. I would not want to do that because you'll spend the entire meal watching your clock and worrying that you'll miss IllumiNations. The only good thing for you is that IllumiNations can be viewed from most locations around WS lagoon, so you won't have to really go to far for a good place to watch.