Batteries can hold a charge very long if they are well insulated and kept cool. A battery's storage capacity is what is lost with age. Over time and after several charge/discharge cycles the potency of the electrochemical materal in the battery
cells become tapped out. The battery slowly loses it's capacity and will discharge quicker.
Also oxidation on the cell plate surfaces can build up, creating a platina of non-conducting oxidation that robs capacity. Fresh electrolyte can rejuvinate a battery, as well as a "Hard Charge", a fast charge from a high current source that can physically boil the platina off the surfaces of the batteries cells.
I bet your battery can keep a charge very well, but you may find it may tend to discharge faster with age.