If factory centerfire cartridges are stored in a dry, cool place with low humidity, preferably in an airtight container, they can have an incredibly long shelf life. Many ballistics experts who have fired tens of thousands of bullets over the years report that they have fired ammunition for 20 to 50 years without problems. On the other hand, I would recommend a more frequent rotation of the defensive ammunition loaded into your transport weapon. Ammunition loaded and deposited in a transport firearm is exposed to temperature variations, cleaning solvents, lubrication and a lot of movement (which, over time, can disable the primers).
The oldest ammunition will probably fire, but don't bet your life on it. Replace your transport ammunition at least once a year. Proper ammunition storage is crucial for the firearms you rely on in an emergency, such as your concealed firearm. A weapon like that could go for years without being fired.
Although most manufacturers recommend not using ammunition older than ten years, prevention is better than cure. After a few years, he uses those old rounds at the first opportunity, maybe practicing a little at the shooting range. The useful life of ammunition depends to a large extent on the conditions in which they are stored. Most manufacturers guarantee that their ammunition will last at least a decade.
That said, ammunition can easily last more than 10 years if stored under ideal conditions. There are countless stories of surplus military ammunition being used many decades after its manufacture. Ammunition manufacturers limit their liability exposure by being more conservative in their estimation of useful life (not to mention selling more boxes of ammunition by encouraging more frequent purchases). I like the armscorr washed copper 36 Rn HP for cheap practice ammunition and it will be about the same or cheaper price as Remington when ammunition inventories meet demand.
Ammunition companies are pushing a conservative message, probably because they don't want responsibility if it doesn't shoot (and, hey, they'd like to sell more ammunition).