Several years, in the refrigerator if there's no load placed on it. If you're actually USING the battery, then its lifetime will depend on the size of the battery and how much current you're drawing from it.
No set answer to that, it depends on the size/capacity of the battery and what it's powering.
No, you can not charge a 3V battery with 12V charger.
2032 3V Lithium Battery
at the store
CR 2032, 3V lithium battery
CR1616 - 3v battery
its round and flat
3V 2 Triple AAA battery's
The battery used from the factory is a Panasonic CR1616 3v. Other companies make CR1616 batteries that will fit as well.
Cr2016 3v
3v battery is used direct current
The difference between the two is the 2032 3v battery is just one battery per package. The 2016 3v are sold in packages of two. Therefore, the 2016 would mean you would have 2 batteries to use. One to replace the battery that has died and one to save as a back up or to put in a second remote for your garage door opener.