Our lunch at Ariel's Grotto last week took about 1:15 in total. We were late arriving for our 11:40 seating and were seated right around noon (don't try this, they generally only hold reservations for 15 minutes past your time). We were done and on our way out shortly after 1:00. We weren't pushed out at all. But they are very efficient in serving you. Our appetizers were out right away and our main course was out very very quickly after we ordered it. As soon as we were done our main course, our dessert was served. The servers are very attentive. We didn't feel pressured to eat fast and get out. But one thing is that the princesses come out, do their tour through the restaurant and then leave, so once they're gone, we didnt' see them again. But each princess did spend a few minutes with Littlenuck, so it's worth it. It's not like they just walk past. They actually stopped and helped her put her crown on her head and talked with her. They likely didn't come in again until the next seating was already in place.
I'm sure if we tried to stick around for a couple of hours they might have said something, but after an hour and a bit we were ready to go.