I've been many times and the short answer is arrive 60 minutes before the show. For the long answer, I'll tell you about two plans I used, one unsuccessful and the other successful.
First Plan (2012 trip, Spring Break):
Enjoyed Epcot all day. Late in the day we walked through International Gateway and boarded the boat for a nice ride to the Studios. Once in the Studios we ate. It was getting cold and my backpack was so stuffed with warm clothes for the kids I was left undefended. Thinking we still had plenty of time I bought a sweatshirt at the gift shop. My master plan was going great until we arrived at the Fantasmic entrance 45 minutes before the show and were told it was full. Would have been fine if we had gotten there 15min earlier. Opps.
Second Plan (2013 trip, Spring Break):
Enjoyed the Studios all day. About three hours before Fantasmic, my dad, sister, myself and my oldest kids (10 and 13) rode TOT. Armed with numerous FastPasses, I let the kids ride a few more times while the rest of us camped out at the shaded tables just outside of TOT. We sat there enjoying ourselves and a tasty beverage or three for the next two hours, all the while keeping half an eye on the Fantasmic gate. As soon as it starting looking like a crowd was forming, I texted the whole crew and we merrily strolled up the ramp and saw the show. Great time!