It depends on what time Wishes is showing when you go. I always try to get ressies for about an hour or so before the scheduled time. The wait staff will not rush you out (like at many of the other Disney restaurants) so you can linger over your meal. We often end up spending 2-3 hours over a meal. Granted when we have the kids in tow we spend less time - usually 1-2 hours. The good news is that even if you can't get ressies for a time when the fireworks are going off, you can return during the fireworks. Just show your receipt and they'll let you up; even if you eat lunch there you can return to watch the fireworks. Also, I've read that you actually watch Illuminations from the catwalk on the other end of the building, but I've not tried this.
Finally, don't miss the opportunity to visit with the sushi chef. I don't remember her name, but she is very friendly. She's a little hard to understand as she's Japanese, but she seems to enjoy visiting with guests very much. Getting up and walking around to see the open kitchen and talking to the chefs during dinner and taking in the view is probably part of the reason dinner takes us several hours!
