Forget shelf life as long as the powder is kept dry and stored indoors. I doubt anyone knows how long powder will last and retain all its properties. I've used fifty-year old powder that worked as well as new and I'm sure others have used older powders than that with perfect results.
I've heard of deteriorated powder, but in sixty years of handloading, I've never encountered any. Bad powder is likely the result of poor storage conditions or containers that weren't tightly sealed.
