I think it depends on what time of the year and the crowds. One November evening, we got there 20 minutes prior and got a seat. Last summer, I believe we were there an hour before (or longer, I don't remember the exact time period other than it was long and HOT!!! ) and got seated with no problems. The show is definately worth the wait!
There were two shows back in the late 90's (in November!). Last June, there was only one show when we were there. Keep an eye on the boards here and they will tell you hours for Fantasmic!when Disney releases the hours.