The rule of thumb is "the bigger the beer, the longer it stays" as well as "the darker the beer tends to be, the better it ages"...granted there are going to be exceptions to this. None of my beers make it to the "aging stage" they empty pretty quick. So I have a barleywein that I made in January and its sitting ina secondary that I wont touch till june. Its OG was 1.089 so I would expect it to age well over the next 6 months. I also have a few ales which I will drink after 1 month in the bottle. I would expect those modest ales (OG 1.048) would last about 5-6 months before starting to get "off tasting". Since we are homebrewers we have to rely on regular ingredients to preserve the beer, ie hops and EtOH, while commercial brews have preservatives added for shelf-life.
-Sometimes I feel like I'm up beer creek without a pint glass-
-May the IBU's be with you-