Alarm Batteries usually last about 5 to 7 years, depending on the amount of use they get. When I say use, what I mean is, how often does your alarm go off and how often does your power go out? The battery serves as a backup power system to your alarm, so it can do its duty whether there's power or not. Most of the time, they cover storm outages of one or two hours a year.
If you live in an area with a lot of storms, they will use the batteries more often and thereby lose their life more quickly. We have had some customers call us with batteries over ten years old.
Unless your alarm is defective, you should be getting a minimum of four years from your battery.