Depends on your group and what you want to see. Our family consist of me, DW, 4 yo DD, and 1 yo DS. I am the only one who rides RnR, ToT, and Star Tours. Here is what we typically get done in 4 hours:
Little Mermaid
Playhouse Disney
Beauty and the Beast
Honey I Shrunk the Kids Playground
Some characters between shows
RnR (I do with FP while family wait for show)
ToT (I do with FP while family wait for show)
Star Tours (I sometimes do while family waits for show)
If your whole group wants to do all of these together it might take you six hours, but could be less if you hit everything right. Each of the shows we see takes about 45 minutes to an hour between waiting in line and seeing the show. RnR and ToT with fastpasses both take about a half hour between waiting in line and riding. You need about a half hour to let kids enjoy the playground. You are already up to about 5 hours minimum including time to move around the park.
Considering all the major things that I don't have on our list (Indiana Jones, Great Movie Ride, Millionaire, Backlot Tour, Muppets, Fantasmic, and the parade) you would really have to pick and choose to get out in six hours. Also remember that if you decide to be in the park later in the day to see the parade and Fantasmic you might miss some of the shows like Beauty and the Beast as the last showing could be as early as 3:45.
Many people claim that MGM is a half day park, but they obviously aren't doing everything. All the major things I listed that we don't always do would take a minimum of 4 hours. Throw in a meal or two and you are at 10 hours easy. Then there are a few things like the animation tour (which is very good) that I didn't even mention.
So........to do everything you would need a full day. To hit some highlights and your favorites 6 hours will do, but realize you won't be seeing everything.