I would budget 90 minutes for BoG dinner if you plan to get a picture with the Beast, because there is often a wait to meet him especially if he goes on break. But depending on how slowly you eat and if you have to wait to be seated, it could take longer.
We visited BOG last year for breakfast and it was about 60 minutes total. This accounts for the time it took for us to be seated, wait for our food (which came fairly quickly) and the time it took for us to eat. I would agree with PP that if you want to get a picture with the Beast, 90 minutes sounds accurate.
When we went last June I budgeted 75 minutes for the meal. We were stuck in that dive for almost 3 hours. They finally brought the check when the girls got up and got in line for the Beast. We were there late in the evening thou. And thankfully it made the girls never again list.