how long should a battery stay charged ?

[smithy318i]

if you charged a 12V battery, then left it unused, how long should it take to discharge enough to not be able to turn over the car?

weeks?
months?
years?

[agent006]

The battery in my 325 went completely dead (unchargeable) in just over a year with the previous owner.

[Martinaston]

The Bosch one i got from GSF had a sticker saying it was to be re-charged every three months while its sitting in the shop, so i'd say that was on the side of caution and would take about four months to go flat and become completely useless if left dead too long. But thats for a new battery, the older they are the weaker the acid becomes so they die quicker.
If your storing one then check the voltage every couple of weeks to get some idea how fast it's loosing its charge and then top it up every six weeks or so

[TOURINGDADDY]

once a battery goes flat the plates buckle and then wont hold charge for half the normal time. once a batt goes flat i buy another 1