I would be wary of going more than a year. The stuff will eventually dry out and you can end up with "Stan's boogers."
I have also heard that climate conditions where you ride can affect the speed at which sealant dries. I cannot remember the specifics, but it would seem logical that if you regularly ride in a hit, arid climate it would dry quicker since it is water soluble.
Anyone feel free to correct me if my supposition is inaccurate.