We just have diner a Goofy's Kitchen and was told when I made the reservation you get 1 hour and 30 mins for diner. But it was not busy when we went and no one said anything to us about having to leave and we were there longer then that.
Just to be clear, when you go in they give you a ticket that tells you there is a 90-minute LIMIT. (at least in my past experiences).
We have found that they have never "kicked-us out", but we are generally done before that would happen anyways. We can usually eat and play with characters in about an hour or so. After that, my boys want to get up and go!
And remember, you don't have to hunt down the characters and stand in line to meet them.... you sit at your table and they come to you. It is much more convenient and less chaotic that way. Have fun!