It definitely depends, and not just among different MTs. I’ve had an hour session end early and gotten some nice face and arm massage action and I’ve had it end just after they do the weird pressing on my groin with a warm wet towel to clean up move that I don’t get. And this is from the same MT.
For my midsized area it really feels like the experience you get can vary wildly depending on how busy they are and seemingly how the MT is feeling that day.
If that is a big thing for you, I’m sure if you’re a good customer and tipper you can get post finish work if you ask. And I’ve seen reports from the old subreddit of people getting multiple pops in the same session.
And if that isn’t something you want and you want to leave immediately after finishing, again as long as you are a good customer and tipper I can’t imagine that being a problem. You are letting them be more efficient and make more money at the end of the day.
After cleaning up unless they say okay and leave they will probably just continue the normal massage. I've never heard of them just playing with it more until time, no. I've heard of mongers who like to get the hell out of there asap bc that's all they went in for. If you are there for the massage and also want additional extras I would go for 90 minutes and say up front that's what you want, so you'd be paying for more table time plus two HEs. I don't leave early unless it's not a good massage. For 30 minutes they usually barely have time to massage the back side of your body. At a jack shack it might be an in and out situation where they make it apparent they want you to leave after, but I've never seen that. You could also try for an independent and ask for GFE if you want stuff afterward.