The battery life is rated in charge cycles, and all our batteries are rated the same: 750 cycles.  This is in reference to to the number of full charges the battery can handle before the ability of the battery to hold a charge diminishes significantly. Perhaps you have experienced this with a cell phone or laptop computer.  

Initially, the device holds a charge well, but over time, the battery simply doesn't last as long between charges. The same thing happens with electric bicycles; the battery is still usable, but  your range will decrease.  750 cycles translates to about 3 years of average use, and this will be affected by your specific riding habits. 

Of course, the battery needs to be taken care of by charging it if is is in storage for more than 2-3 months, during which time it should be kept in a temperature controlled environment.

After 750 cycles, performance is still about 80% of original, and about 40% after 1300 charge cycles. If you drained and recharged your battery EVERY DAY, that would translate to about 4 years and over 20,000 miles!

Here a great website if you wish to learn more about Li-ion batteries ->