It varys from battery to battery, and it also depends if its a Generic battery or an official one.
Generic batt. are half the price but normally only last half the time.
A Batt. should last around 2 years but ti depends how you charge it?? If you let it run right down then charge then Batt. will last alot longer then if you keep recharging when its half empty. but to be honest I dont know anyone that let the batt. drain everytime, I try to but get into a habit of plugging it in when i go to bed regardless of how much batt. power is left.
Hope this helps