Depends on the beer's alcohol and hop content, as well as your own sanitation procedure. Finally beyond that you've got the fact that oxygen can have a slow burn effect such that oxygen introduced at barreling does not have a deleterious effect after the first few months but attracts a progressive vinous, winey ultimately nasty character. It's still "drinkable" but not necessarily palatable. Sorry for the vague answer but it will keep 6 months whatever it is if you've observed the most rudimentary sanitation standards.
