We went to Sanaa for lunch on our AK day on our last trip. We were at the bus stop an hour before our reservation. We got to the restaurant to check in about 25 - 30 minutes prior to the actual reservation time. This worked out because there is a great viewing spot for the animals close the restaurant so we had time to relax and watch the animals. After a pretty leisurely lunch, we headed back to the park. All in all, we were out of the park for about two and half hours. For us, it was a good break. Animal Kingdom can be a hot park, it feels like there is less shade than some of the others. So we appreciated the time to rest.
Side note, I highly recommend the bread service with the accompaniments! It is really amazing!!