Totally depends. Weekend day where I'm covering for my partner and the goal is just to make sure everyone gets seen? Could be as little as just a few minutes if no active issues. If I'm counseling, I could be in there for half an hour.
I usually end with "what questions do you have for me," since so many people don't ask questions they have unless you ask them. Then I say something like "have a good day,", "keep up the good work in therapy!", etc.