How long will bottled beer last?

BrewYourOwn magazine (Mar/Apr 2011) just did an experiment on how storage temps affect home brew.

Their main finding was just as Denny said: high temps will degrade the beer and cause off-flavors. In this experiment the "high temps" were >100° (like in an attic during the summer) for weeks at a time.

"Room temp" storage was almost indistinguishable from "Cellar temp" storage.

Like Brewchez and Denny have said, avoiding high temperatures and light are the most important parts. I've found that a lot of my "muddy" beers have cleaned up after around a year of storage in a dark ~65F closet. Case in point, I had a pumpkin ale that was neigh undrinkable two years ago, and almost an entire case remained in the closet. This fall I dug it out again and found that all the "pumpkin pie spice" flavors that they always talk about had melded into something surprisingly drinkable. I wouldn't go more than a year or two for low-gravity beers, but I'd say leave it in there, forget about it, and you might be surprised by how well things turn out.

It depends on the beer and the storage conditions. To start, the stronger and hoppier the beer, the longer it will keep. For instance, a hefeweizen won't keep as long as a barleywine. The temperature is also important. Generally, cooler is better. The main thing is to avoid excessively high (85+) temps. Avoiding temp swings helps, but it's mainly high temps that are your enemy. I've kept 1.100+ bbers like Am. BW and RIS for 10 years at normal room temps and the age just improves their complexity. Of course, you must have excellent sanitation, too.