If the battery you mean is the button type clipped onto the motherboard then I would have thought at least 2 years life. There is almost no drain on the battery it simply maintains a set voltage in the sensitive guts of the thing. The battery doesn't actually drive anything.
If I am talking about the right thing, you should be able to pick up a replacement almost anywhere from computer shops or even a local jeweler who may use that type in his repairs. The only important thing is to be sure you buy the correct battery ( type and voltage useually engraved on the battery) and use gentle care to replace it on the mobo. Avoid handlig it or wear thin gloves.