Not sure about dinner, but we did lunch (family of 4) on Friday and it was about 90 minutes. Of course this is with 2 smaller children (DS4 DD2) and we lingered a bitto ensure we got photos, as it was a beautiful day and the light in the room was just awesome.
By the way, the food really surprised me - I had heard that it was so-so, but we thoroughly enjoyed it.
It was quite an experience overall! If you've been able to score reservations, make time for it!