To keep your battery from going completely dead you should at least start your car and let it idle for 10 minutes at least every 2 days.
It's better though to just get in the car and take it for a drive even if you just cruise around town with nowhere to really go.
Driving the car around will not only keep your battery alive and well it will also extend your tires life since when you get the tires rolling they'll stretch and won't be prone to rot away as fast or crack.