People always say that, but to really see the major things would be a problem in 4 to 5 hours. Unless the park is absolutely DEAD that day.
Lights Motors Action is a good 1 hour experience in terms of getting in, seeing the show and getting out. Indy is about the same, so plug in Beauty and the Beast, and you are looking at roughly 3 hours minimum just for shows. Then factor in Muppets, Great Movie Ride, Star Tours all on standby, thats a good 2 hours there. Then throw in ToT, RnR, TSM, being optimistic, that's 2 hours. Then some of the smaller shows like Mermaid, Idol, the parade, some meet and greets - easily a couple of hours there. factor in food, and then of course Fantasmic. It's a 7 to 8 hour day to see most of what is on offer on a typical day in my opinion