As I understand it, forever. However, there is some debate about certain additives that are used in the oil. The ones under debate aren't used in every oil, so I see this as a non-issue.
Realisticly, how many years supply do you plan to store? I would say right now I've got a 1.25 year supply for my Ranger and a 4 year supply for my Oldsmobile. I have never even considered or thought of the oil going bad while being stored in my non-climate controlled garage.
I would worry more about what the oil would cost next year as opposed to if the oil I bought now is going to still be good next year.