As mentioned by Train2, battery life is not measured by time, but by the number of charge/discharge cycles. The standard is about 300 charge cycles for most inexpensive, Ego type systems. At about ~250-275 charges or so, you should notice a decrease in the battery's ability to hold a charge. This is the point where you dispose of it properly and replace it.
Batteries should be considered a consumable item. At some point, they will need to be replaced.
It's always a good idea to have a backup, and a backup for your backup.