I have some old .380 ammo that I've had for 10-15 years and I wondered the same thing. The stuff recently fired OK, it just made the barrel a little dirtier from the patina on the brass when I did finally use it. Likewise, I've acquired Korean War surplus rifle ammo that fires just fine 50 years later (that stuff was sealed for storage pretty well though). I also recently sold a leftover box of S&W short .38 rounds at the shop that looked like a package from the 60s and the guy said it worked fine. So, my belief/experience is that as long as you keep it dry and it's not visibly corroded, you should be OK for a couple/few decades, even if it looks cruddy.