Maybe a good rule of thumb is that at least 10% of a session should be reflection.
60 min session = 6 min reflection
3 hour session = 18 min reflection
3, 10 hours days = 3 hours reflection
I think it’s important have to have little heuristic like this because without it, you might just let the time slip. You might run the session all the way up to the last minute. Even if 10% isn’t perfect., at least it’s a good place to start.