I personally cannot state an expiration date but with no light hitting it and it remaining at rest I would say that it can only get better over time as long as the keg is purged of any oxygen and always has c02 in there. I would say it could last years. But I may be a little naive as I started kegging just a little while ago.
Refrigerated, it will last a long time, months to even a year or more. Even stored at room temperature, I've had kegs that were good a year later. The hop flavor in IPA's will tend to fade over time but otherwise you should be good.