Its about 3 litres short between when you fill it with the engine off and then top it up with the engine running as it comes up to temperature. Its only a few minutes with it idling, but I still don't like it since if you drove it 3 litres short it would kill the box in pretty short order.
I've never checked to see how long it takes to cool down tbh. If 2 hours will do it then it is practical to flush like that
