10 minutes. 30 tops.
But keep in mind, there are 2k+ participants and the average attention span is less than 9 seconds, so get to the "cool parts" as FAST as you can! =)
Great question!
Longer game times mean longer dev time to polish, balance, and test...not to mention any additional features that may need to be created to extend the fun. Longer game times also encourage devs to bury their "cool parts" later in the game and/or run out of time to make the whole experience optimally fun.
Given the short dev time, it's better to make a tight, highly polished short experience than a lack-luster longer experience. (Think of the award winning game "Portal", which only ran about 2hrs. The game won multiple top awards as "Game of the Year", utterly annihilating the flood of 40-60hr first-person games that came out at the same time.)
But, in the end, do whatever your game requires. Every game is has its own needs...and some need longer run times. Remember, I'm not writing rules, I'm just offering suggestions based on my collection of knowledge and experience to help those who ask for my insight on how to best optimize their experience for this jam. Everyone is free to take my advice or not. Either way, the important things are just to have fun and learn something! =)