It completely depends on the style of beer. Some styles such as Belgian Wit or IPA are pretty blah after a few months. Other styles like Russian Imperial Stout can sit in the bottle for years and be just as good or better.
Yeah,it's more like how long they can sit before hop flavors/aromas start breaking down into bittering. And the flavors get a bit maltier/weaker. It could be 2 months to a matter of years till this starts setting in. Big darker beers will last longer than lighter,mid-gravity ones.