But saying that beer actually expires is a bit misleading. It doesn't actually spoil or become unsafe to drink. It will just start to taste flat, flavorless, and unappealing.
How long a beer stays fresh and how fast it becomes unpalatable depends on the specific beer. Most craft beers stored at a stable temperature and out of the light will be good for about a year before starting to turn. Look for a bottling date or an expiration date on the bottle's label, as well. Many breweries have started printing this information for our benefit.
Some beers, particularly bottle-conditioned ales with an alcohol content of 9% or higher, can be aged for longer than a year. They experience the same breakdown in flavor and structure, but this has been calculated to actually improve the flavor of the beer. Hop bitterness fades away while malts take on a deeper, more honey-like flavors. Other flavors emerge that may not have been perceptible before. It can be a fun experiment to put away a few bottles of a strong beer like a barley-wine or a barrel-aged stout, and then open a bottle every few months!
Related: The Real Truth About Skunked Beer