For supercuts, I've found my audience retention rates drop on 3-5 minutes (depending if it was a good ep or not). On that basis, for me personally, I judge if I think the video was one of the better ones, then put it around 3 for a kinda eh one, 5 for one of the better ones, and 4 for the ones in the middle. Of course, sometimes to keep continuity and fluidity of a story based game for example, this doesn't apply as much, because the audience has to be able to keep track of the story. Those types of super cut videos can go way past 10 minutes easily.
Just depends on the video if you ask me.