If it were me, I'd drink it after 1-2 years. It's a big beer and worth cellaring, but dang, I don't know if I'd go much longer than that. It would be nice to have maybe 3 years' worth of KBS to do a vertical tasting with.
RIS and Barleywine are probably the only beers I'd ever want to cellar. I've even had very good barleywines that were really nice after only a few months of aging. It jus tdepends on how hoppy you like your big beers, and how much you enjoy the aged flavors.