It depends on the gravity, the hopping rate, and the final alcohol content, but even with moderate strength beers (4-6%) you generally get a greater shelf life with bottle conditioned beer than with commercial beer. Generally speaking, a well-kept bottle will last two or three years at least (except for weizens, which tend to deteriorate quickly for some reason and rarely last more than 8 months). YMMV, though. About five years ago I had a pilsener I brewed in 1993 (partial mash, IIRC), and it was still in good condition, despite less than ideal storage in a drafty garage. Stronger beers could last up to twenty years with good cellaring.
Note that bottle conditioned beer will change over time, sometimes for the better, sometimes for the worse. Generally speaking, hop character (especially aroma) dissipates quickly, with aroma usually mostly gone after 8-12 months and bitterness slowly decreasing with age. OTOH, esters tend to mellow and mature with time, and malt character will often come to the fore, especially with high ABV beers.