A cell phones battery should easily last around 3 to 4 years depending on how much you use your cell phone and if you use a lot of apps that drain your cell phones battery faster.
If you let the battery discharge enough before plugging in the charger and then fully recharging the battery again from the battery being nearly drained then the cell phones battery will last longer than just plugging the cell phone into the charger when the battery gets a little low.
Most times people will just replace their entire cell phone with a new cell phone by the time the battery needs replaced and some cell phones have the battery sealed anyway where you can't remove the battery and replace it.