We tend to always go during the summer, and we've had a different experience than some other posters.
Once the sun starts to go down (around 7:00ish), we tend to notice that the attractions do clear out. We've walked onto MuppetVision3D, Star Tours, Great Movie Ride, Sounds Dangerous, and often caught shows of Indy and Little Mermaid without any problems.
However, lines are almost always long for TOT and RNRC. That being said, if there are two Fantasmics the night you're going, the park really seems to empty out around 8:00 even for TOT and RNRC. We've always felt like we've had the park to ourselves if this is the case. Then, we go to the second Fantasmic only about 30 minutes prior to the show (this second show is significantly less crowded being that it usually starts at 10:30).
One thing to remember, though, like a previous poster said, the shows don't play all through the night, so you may miss Who Wants to Be a Millionaire, Beauty and the Beast, Indy, or Little Mermaid.
Now that I've bounced back and forth in my opinion, I would say that it would probably be enough time if there was a second Fantasmic; if there's only one, I wouldn't do it (then again, MGM is our family's favorite park, so we're kinda biased).
RyMickey