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As for absolute longevity, I have a large stock of Turkish military 8mm ammo that has mid-1930s headstamps. Consevatively, this stuff is 80 years old and who know how the Turks stored it before it was imported by Century Arms in the late 1990s (2001-2002 I was buying this stuff for I think, $7 for a 70 round bandoleer and I bought a LOT of them). I have had no misfires and it prints to point of aim out to 100+ yards with all of my Mausers. The key factor is that for all of its life, it appears to have been kept dry and there is no obvious corrosion even though there is some age discoloration of the cases. All of it is Berdan primed. Thus, I believe that ammo will continue to function for longer than humans as long as it is kept dry.

As for your carry ammo, I would not really worry about the heat in the car for the periods you are talking about. For me personally, I try and shoot my EDC at the range at least once per month and the first magazine is the one that has been in the firearm that month. After that first magazine I switch to my reloaded range ammunition. I buy may carry ammunition in 50 round boxes so will go through a box about every 6 months. It is worth it to me to do at least a little of my practice with my carry ammo even if I have to buy a couple of boxes per year.