Most beers in the 1.045 - 1.065 range taste their best starting about 2 months after brew day, IMHO, although they may be perfectly good before that.
Lower gravity beers mature a little quicker, higher gravity beers may take a little longer.
Beers stored in reasonably stable temperatures below, say 70 F, will last a long time: easily 6 months to a year (probably much longer). Many beers will be past their prime at that point, though.
Refridgeration slows down the aging process considerably, which is why you shouldn't refridgerate ales for normal storage unless you want to store them a long time.