I usually let the black tank rinse run for about 5 min to get it good and clean. I can basically tell that it's clean by looking at what's coming out of the sewer pipe (it should be just a steady trickle of clear water) and/or when the black tank sensor shows empty. If th black tank sensor doesn't show empty that tells me there's probably some crud stuck inside and a run the spray for longer or I dump some ice cubes in the black tank on my way home. The ice cube trick worked great on my Tango, but I've never had to do it on the Premier.
We usually stay with full hookups so there's no rush using the black tank flush. I wouldn't use it if I were at a dump station. I'd just use the ice cubes technique on the way over to the dump station.