Welcome to Android Central! That's going to depend on how heavily you use the device. Batteries generally have a lifespan measured in charge cycles (i.e, discharging to about 40%, then charging back up to 100%). I believe the average battery will start to show some deterioration in performance after around 400-500 charge cycles--but realistically, you may not notice anything until it's at least 2 years old.
Other factors may influence this as well, like extreme temps. If you have a habit of leaving your phone in a hot car, the high temps that can occur can cause the battery to discharge faster, and may also shorten battery life if done repeatedly.