I have no doubt that 30+ year old ammo will fire being stored under unknown conditions, and have proven it with a couple rounds that were purchased around 1983 or so in my grandpa's Winchester lever action. I still have more in the box, and will try it in a few years to see what happens. However....
...when it comes to self-defense ammo, I'm not going to go with anything older than 6 months in my carry gun. Six months covers a minimum of 2 season changes, and depending on where you live, can have widely varying levels of humidity.
Plus, carrying SD ammo in your gun on your hip subjects the it to even more temperature and humidity changes, particularly if you live in a cold winter climate. You go from your home, to your car, to the store or whatever, and then reverse the process on the way home. In one trip, you have potentially subjected your ammo to several rapid environmental changes in fairly short span of time.
Would quality SD ammo survive this process repeatedly? Almost certainly. I would wager that even cheap range rounds would be fine, too.
I'm probably being a little anal about it, but I swap out my SD ammo every few months. In the grand scheme of things, a box or two of primo fresh SD ammo twice a year is worth the peace of mind to me.