If you're not doing the dinner package, you typically need to be there an hour to an hour and a half ahead to get a decent seat. It depends on the time of year, too. There are slower times when you can get away with 45 minutes ahead. If you go when there are two shows on one night, if you go to the later show you can be there as late as 15-30 minutes ahead and do just fine.
A lot of people grab something from a food stand (there are several on Sunset on the way down to the theater entrance -- the entrance is just past the Beauty & the Beast theater) and take it in with them. You can get a seat and eat your dinner while you wait. There used to be a pre-show, but now they just play Radio Disney music and inevitably somebody starts a wave.