The old potassium chlorate primers would last indefinitely like black powder does. The mercuric primers would deteriorate, sometimes in just a few years. It is one reason the military used chlorate when mercuric primers were still the only non-corrosive primers available. Lead styphnate primers have been adopted by the military (your first clue that they are at least very reliable and I haven't encountered a military stockpile time limit for them), and I have never heard of properly stored ones breaking down, but then, I don't hear everything. The new Federal catalyst primers, however, use nitrocelluose, so they will have a stabilizer, I assume, and will eventually age out.