At least many months (assuming proper storing, cool and dark). I think some people have even said they've had beer a year or two old and it was still good. I wouldn't worry about it going bad assuming you're storing it well.
EDIT: It also depends on beers. Hefes don't store as long, I guess a rule of the thumb, the dark the longer it will last. I'm not sure exactly how true that is, maybe someone can chime in.