In the Dec 07 Zymurgy Charlie Papazian reviewed bottles of homebrew going back to the first AHC competition that he had stored, and none of them went bad, some had not held up but most of them he felt were awesome...We're talking over 20 years worth of beers.
It's going to really depend on the style and the recipe...Barleywines may not even come into their own for years and if properly stored may be good for decades.
A hefe on the other hand should be drunk really young..
I've tasted year old bottles of beer, and none of them had gone downhill either.