How long is a Dell Inspiron 1525 battery supposed to last?

So i got my Dell Inspiron 1525 for Christmas last year. About a week ago every time i turn it on there is a warning that says "Your battery is able to charge normally, however, it is reaching the end of it's usable life" So now i have to keep it plugged in all the time or it will die faster than usual. The battery is only 11 months old, and it was brand new when i got it. Is that the normal battery life for it, or was it supposed to last longer?How long is a Dell Inspiron 1525 battery supposed to last?less than 1 and a half also depends how many time u charged it