It varies. I've had cars sit for months and fire up without a problem. If your car does sit a lot you're best off just picking up a "battery tender". They're not too expensive (but I do stay away from cheap generic stuff for things like this) and their job is to keep a charge on the battery.
Running it once in a while for short periods just leaves moisture in everything (engine/exhaust internals) and increases wear for no real good reason. A decent length drive allows everything to get up to full temperature. Its also the case that while an alternator does keep charge on a battery its not a battery charger. If the car sits a lot, & you start it for 15 minutes you probably just about replaced the battery power used when you cranked it up.
So I'd get a battery tender and only take it out when I know it will get a good "work out."