It really depends how much you use it, what functions you have on/off etc.
If you always have the sounds on, and are ALWAYS pushing buttons unnecesarily, the battery will run out quicker than if you dont.. I dont know the amount of time it will take on average.
:furawatchi: i havent changed battery and i got mine in janurary so mine might last only until june i dunno so they could last for ages if u dont always push button or sumthin like that :furawatchi:
i had the sound of since i first got it , and it lasted exactly 2 months!!!! the batterys died just now!!! i got my v3 march 11 and april, MAY ,thats so kool. (i didnt have to change the batterys) TIP: if you turn the sound of the batterys will last longer!!!
My tamagotchi has been on eversince the first week V3's came out, and in May, it's still functioning. It really depends on the quality of the battery it has.