But, if there's anything I've learned in my years of doing random gamejams... short and sweet sometimes works the best. Shoot for a 15 minute experience, but if it's 20+, that's cool too. But you don't want people playing a game for too long, especially if it's not polished, full of bugs, etc., respect their time.
If you aim to do something short, you can polish the heck outta it.