For how long should a battery retain its charge when disconnected from the car?
It depends on how old your battery is to begin with. If you have a new battery, and disconnect it, it will hold good charge for 2-3 months. if the battery is older, it will approx. hold charge for a month or month and a half. But if you take the battery OUT of the vehicle, do not set it on concrete, the ground, or steel such as a steel plate. It will draw the charge faster and drain the battery in about 2 weeks. If you have to set it on the ground make sure you have a piece of wood or rubber matting. Just as long as it is non-energy conductive.
As long as it doesn’t sit on the ground (or concrete, blacktop) it should be good for maybe six months, as long as it has a charge before it sets. I would open the cells, fill with distilled water, charge it, and it could be good for six to eight months.