However much you think you can get done in a weekend... cut that down by at least 50-90%. If you've never made a game before, you'll be surprised how much time things take to get finished --- AND how many little things you'll be working on that you didn't even think about.
So, as 7Soul said, make it short. Keep your idea simple and just try to get something playable. Games are a lot of work, but are very, very rewarding when you do get things together.