We've never gone to a Fantasmic show more than 30-45 minutes before the start of the show. At 30 minutes once they were just announcing that it was going to be standing room only, but we managed to find seats. We are normally there during a time with average to slightly below average crowd levels.
On our most recent trip, we were able to grab FP+ for Fantasmic about an hour before the show. We wanted to sneak in one ride on Rockin' Roller Coaster so we took it, and we were able to walk in a little bit later than we would have otherwise.
The wait doesn't bother us too much though. We sit and snack and chat with the people around us. It doesn't feel like a terrible wait when you are occupied.