I would give yourself plenty of time, budget for a couple of hours and if you are done quicker than that, you can always do something else afterwards.
We spent at least 20 minutes downstairs before we were seated, we were waiting to see the Fairy Godmother, but unfortunatly the line was long and we were called upstairs, so we left her. We had our meal which took an hour or so, and then came back downstairs, and the Fairy Godmother was still there, so we waited a while longer to see her. The family behind us were trying to run to a strict schedule and after waiting 5 minutes or so, they got impatient and left - what a shame for those kids - we waited a while, maybe 10 minutes, and then had the most wonderful visit with the Fairy Godmother. The reason the wait was so long was because she was talking quite some time to talk quietly to each child, they all had a really lovely personal visit, a photograph, and then she gave them some pixie dust. It was probably one of the most magical experiences of our vacation, and if we had been worried about being somewhere for a particularly time, then we would have missed it.
Bev