If beer is stored in the refrigerator, it can last for up to two years without going flat. Beer stays fresher longer if kept in the refrigerator. If beer is kept in a storage area or pantry, it may become stale six months after purchase.
When a beer can is opened but no hissing sound is heard, the beer has probably gone flat. Foam is also a really good indicator of whether a beer is still fresh. Beer should be kept as cool as possible and in a very dark place in a home to maximize its ability to remain fresh.