Illuminations is usually at 9:00. I would get my reservation for about 7:30 so that you have time to eat. If you are lucky enough to get seated outside by the rail you can enjoy a leisurly dinner and finish up during the show. If not, you will have enough time to eat and get out to the patio to view the show.
My husband and I always watch Illuminations from the Rose and Crown, it is a tradition for us!