Ok, my estimated rough timeframe is:
- Graphics (from Asset Jam) - 30 minutes
- Music - 30 minutes
- Sounds - 15 minutes
- Menu - 15 minutes
- Game(play) 90 minutes
It's about getting practice under time pressure! If you go over the 3 hour limit, it's fine, still submit your game! The real benefit is if you do it regularly - you'll get better at focusing, planning, execution and you'll have a bunch of prototypes you can then choose to develop further or take that experience to build something bigger! :)