If you are ok sitting off to the sides - you can get in line 45-60 minutes before. If you want to be closer to the center - you need to be ready to enter when they open. I usually get in line at least 15 minutes before they start letting people up.
If there are 2 showings, you can go in a little later for the 2nd show. There is more seating because I don't believe they hold sections for The Fantasmic dining plan.