Battery Life when mobo unplugged (CMOS Check Sum Error)

I just bought a new Abit GA-8I915P Duo-A (2 days ago)

I left it unplugged for ~8 hrs, and I recieve a CMOS Check Sum Error.
This means that my BIOS settings are gone.

I went back to the store, and they said it's normal, and that when the machine is unplugged, the CMOS can store the settings for as long as 3 to 4 hours.

I'm skeptical.... I think battery should hold my BIOS setting for at least a couple of days.

How long should the battery keep my setting when the computer is unplugged?


Thanks!