If kept dry and stored in the original container you should have a indefinite shelf live. I have seen it go bad though. One thing that will turn it bad is to haul the jug around with you to the range, in the car, the cabin, etc. Measure out a smaller amount in a powder horn and take that to the places you need to go. And work from that. It will keep the powder in the jug's life much longer.

I've shot off powder over six years old and it shot fine.