Overall, I agree with what everyone else has said. How often you need to stop will depend on your own needs for fluids/fuel, which can be heavilly influenced by terrain, weather, etc.
On an organized ride, one other factor may be how the organizers are managing the ride. I was at a ride last weekend where you were requested to check in at each rest stop to help them manage where folks were on the course. So at a minimum, you were off your bike for a minute or so to log your rider number, and if you're off anyway, you might as well stretch a bit and top off your bottles. I might otherwise have skipped at least one of their rest stops; but it was a well run event with well stocked checkpoints and freindly volunteers, so no complaints from me on how they run their event.
