A car battery that sits in the car for a month without charging will begin to go down within the month if the car is not started and drove around for 20 to 30 minutes at least once or twice per month.
If you then allow the car to sit longer than 2 or 3 months then the cars battery will be discharged fully and may no longer be good anymore.
So if you don't drive your car often you should still at least start the car once per week and let it run for 10 minutes.
It's best to take the car on a drive for around 10 to 15 miles at least once per week as well or once per month to keep the tires rotating so that they do not develop any flat spots on them.
Letting the engine run at least every a few weeks helps to keep oil on the engines internal components which also prevents them from drying out and possibly rusting inside and seizing.
An engine that sits too long in a vehicle can sometimes seize up because of the oil draining completely off the parts into the pan.