[Al Skierkiewicz]

This is a hard question to answer. We have batteries that have been around in excess of four years that are still OK for practice. Funny thing about SLA/Gell cell batteries is that after repeated charge/discharge cycles the electrolyte (the gelled acid) starts to evaporate and the battery actually becomes lighter. Compared with a new battery of the same type you can tell by holding one in each hand, the lighter is the more used battery. If you have a battery that does not have the capacity, terminal voltage or weight of a new battery, it's time to recycle it.
Always remember to portect the terminals during storage and transport of even a damaged or old battery. Any metal coming in contact with both terminals has the potential for a catastrophic failure or fire.
Good Luck All