This will be different for everybody, but the honest answer is – as long as is needed. Dump your black tank then your grey tank, then clean up. If you always fill your black tank with roughly 10% water after dumping and use enough water when you flush, dumping is quick and easy (Of course, the time it takes depends on tank capacity and so on). If there’s not a line of RVs waiting behind you, go ahead and flush your tanks if needed. But I typically wait to do this at campgrounds with full hookups.
