We saw this show for the first time last month w/ our DD (she is almost 3). I believe the show lasts about 30 minutes. We actually got there 5 min late, but they CM's still let us in and we sat in the back. Be aware that there are no actual seats or benches in the audience, although there are benches in the back near the walls. You sit on the floor. Even though we were in the back, the space is not actually that big so you still have a great view and for us it was better b/c there was more room for DD to stand up, jump up and down, dance, etc. Lots of the kids do this, and I think they are encouraged to.
It was a lot of fun and DD absolutely loved it, as she knows all the Playhouse Disney characters. Definitely a must-do for the little ones especially if they watch Playhouse Disney.