Participating in game jam also helps you to learn fast decision making. When jam is about to end and your game is 80% complete, you have to make quick decision to ensure your submitted game is complete.
So my suggestion if you're not sure about joining game jam yet, is to join game jams with long duration: one week for shortest and one month for decent time. Make sure your game is COMPLETE before submitting. This small suggestion should be enough for you :).