Totally depends on the car, age of the battery, and temperature. I would check with the dealer or service to see if there are issues (our car had a recall because some models (not ours) would go three days, then kick in the onboard computer, then kill the battery in another two days). Even if you have to jump start it when you get back, there is probably still enough power to keep the onboard electronics running for a very long time, even if it won't start the car.

If you are talking about months, then consider a trickle charger to keep it topped off if you plug it in, if not there are solar ones for very little money.