This is a tough one. I gave my high school students Level 21 as homework, because it is very guided in walking through the Game Design process - then Lesson 22 says 80-200 minutes... this is challenging because the more time you give and equally push and have constant check-ins, the better the final product. I used the final game as the semester project so they had to be completed by the end of the semester, and ended giving about 2 80 min blocks - which some students came up with great stuff.
The less time you have, the more focused you can push them to complete, but with an understanding that they can come back to the project if they'd like to.
Overall my time was too much, I had some high school students that didn't have a lot of drive (they had already gotten into college) so I had to keep on them for a good product, but my "rock star" students make some really impressive games.
Looking forward to hearing from other people,