I've hinted I'd like a third round when I have had time left on hour appointments - pretty consistently get thumbs down. Most, after second shot, pretty clearly are ready to quit. I've basically gotten pretty good at pacing so round 2's load gets shot with about 5 min left in the hour.
The funny one was little spinner Jenny (at Asian Heaven now, I think.). Round one was CIM, then round 2 I pounded her pretty hard in multiple positions. Had about 15 min left. I was getting hard again and said how about one more? She just giggled and said "no way!" I didn't want to hurt the poor thing, she had half a day of work to go!
Now, if I'm paying for 2 hours, I make it clear up front that it's a cum-a-thon. They can then accept or decline. They usually allow access to all three entry points in that type of session.