A lead- acid battery will self-discharge at about 5% a month at 60 degrees temperature. The colder it is the slower the self discharge…That’s if there is no load on the battery, as in battery disconnected from the car…Today, all cars have a SLIGHT draw on the battery because some circuits are always on…Each car is different so answering your question is difficult…Most cars can still start after sitting for 3 months…After that, luck starts playing more of a role…