One gunpowder stability test is to put gunpowder in a 150 F oven and see if it fumes red nitric acid gas in less than 30 days. If it does, the gunpowder is unstable and scrapped.
Storage in hot conditions, like constant 150 F car interiors, will age and deteriorate ammunition to an unusable condition in months. And I mean unusable in the sense it will blow up your gun. I would recommend replenishing ammunition at the end of the year.
If I'm not shooting, I'm reloading.