Corrosively primed Warsaw Pact ammo should be "factory-fresh" for about 50 years after it's manufacturing date, and probably still working in the 80% category after 100 years.
Of course storage conditions would play a part here.
I have CIL primers purchased in the early 1970's are still working just fine, and the reason that the Warsaw Pact stayed with the corrosive priming mixture was that it is more stable in long term storage than the non-corrosive stuff like we use.