Freshness period: The drinking window
The length of time it takes for a beer to become stale (a papery note, dulled hop character, or other off flavors) is determined by the alcoholic strength and hopping level of the beer. Both alcohol and hops help preserve beer. Thus hoppier, stronger beers keep for longer. Typically, the freshness period for a lager is four months; for stronger craft-brewed ales, five months. High-gravity, high-strength beers such as doppelbocks typically carry a six- to twelve-month freshness period. All of the preceding assumes proper handling of the beer.
How can you determine the "drinking window" of a beer? It depends on the dating system used by the brewery. Taking a typical example of Boston Beer's Samuel Adams brands, the freshness period is the time between shipment from the brewery and the freshness date, or "consume by" date, marked on the label or capsule. In the case of a beer with a "born on" date (Anheuser-Busch products, for example), the freshness period is approximately four months after the date on the label.