Usually the rule of thumb s replace when they are down to holding 80% of capacity. If your using a poor charger such as a 15 minute or on made by rayovac or radio shack or any other company that wants to rip people off then 80 % could occur at less than 100 cycles. A good battery with a good charger should last a lot longer. I have batteries that are several years old now that have just moved into kid toys.