As the days will pass the battery performance will decrease.It all depends on you whenever you feel the battery back up is very low(approx to 15mins) you can change the battery,remember "changing the battery is all upto u,there is no such hard n fast rule, if u dnt wish to change then also its upto u".
Even my laptop's battery is 3yr old n it gives a back up of only 10mins,I haven't changed it till yet,I just plug in the charger n uses the laptop.