All car batteries will discharge over time, even if they're not fitted on a car. It's a question of whether or not the rate of discharge is acceptable. If the battery goes flat over a couple of weeks then there's probably nothing wrong. If on the other hand it goes flat after a couple of days then there certainly is.

In any case it's good practice to keep your battery topped up with a battery charger. Charge it overnight once every couple of weeks when you're not using your car. If that's not possible you're going to have to accept a new battery will almost certainly be required once you start using your car again.