Let me preface this with the admission that I really don't know what I'm talking about... but here's my $.02...
People shoot up mil surp ammo that is 50 years+ old all the time. No real issues (other than a slight "warming" of the loads).
I know of a guy who took a big Alaskan moose with an original box of factory .33 WCF (well, one round out of the box)... don't know age but was early part of the century.
My guess is that it varies by manufacturing technique, storage conditions, type of powder, type of primer, phase of the moon and price of beer in Bolivia... but the stuff will probably be good for longer than you can keep it around without shooting it up!