Battery
 

How long should your battery stay charged
when you disconnect it?

depends on the age of the battery, whether it was fully charged when disconnected and whether it has become sulfated.
you need to have the battery load tested to determine if it is still serviceable.

I disconnect my Corvair battery every year for about eight months during the winter. Leave it in the car in cold Michigan storage. It starts right up every spring. Never put a meter on it to see the voltage. I suppose I will when it doesn't fire right off. So if you have a battery that is loosing volts it's probably over 6 years old or it's got a problem. If you have a warranty get it exchanged before it one day leaves you stranded.

Weeks....Remember the battery is a chemical reaction...depends on temperature and also the SCRUPULOUS cleanliness of the top of the battery.
Depends on condensation and temperature and humidity fluctuations.

unkahal

5% per month. So if you want to keep your alternator alive in the spring (and not burn out fusable links), you should charge your battery before firing up the car, but it should last a year if it was disconnected fully charged.

How long does a battery last on a shelf at Walmart or Costco? Probably months, or longer.

Battery was fully charged, its about 1 year old
it lasted about 3 months

Take the battery back to where you bought it. Some places will requirer them to charge it and then load test it. If it fails you get a new battery. Some will just replace. If it drops voltage in 3 months it's got a problem.

Use a QUALITY battery tender/charger......they are work their weight in charged batteries which last much longer than neglected batteries.

Yes that's all i use

As with any manufactured product, sometimes 'lemons' are produced. If your battery doesn't meet its life expectation, and it has a warranty, take it back.

Speaking to your initial question, there are many variables that influence the length of time a charged battery will remain charged, once disconnected from a charging system. What material the battery is placed upon, being one of the most important. If you place a battery on a bare concrete floor in a relative moist environment, you should expect much more rapid discharge than if it was in an arid environment. The level of moisture in the air and if the air has some measure of salinity (ie, ocean-air) can influence discharge rate.

If the battery is to be left disconnected for a lengthy period of time, sealing it [electrically] from the environment might be your first consideration. But, the best alternative is to keep it on a 'smart' trickle charger which will not over-charge it.