Hi,
I believe the show is around 45 mins. long.
Depending on where you sit, because of the mass exodus, it could take a while to exit the Fantasmic area, and then the park itself.
It actually (according to my MGM guidemap which I bring home with me every year....yeah I'm a freak...I know it) is a 25 minute show. As the previous post mentioned it could take quite a while to exit once the show is done.