Cell phone batteries should easily last at least 3 years and most times people just replace their entire cell phone with a new model by the time the lifespan of the cell phone battery is up.
You can extend the life of your cell phone battery by letting the battery get completely nearly discharged before plugging the charger into the cellphone to charge the battery back up.
But I don't do that since I risk having a low battery when I need my cell phone so I keep my cell phone constantly plugged into the charger when not using it or when I'm at home.