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. 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. Ammunition breaks down with age.
More specifically, ammunition will lose its effectiveness over time. There can be several reasons why ammunition breaks down, such as rusty bullets, gunpowder that does not burn, corrosion on the housings due to moisture, etc. It is recommended to replace the ammunition every decade or so. Practically the only way ammunition will spoil is if it gets wet or exposed to enough moisture for moisture to get into the primer.
Of course, that takes a while; there isn't much room for water or moisture to pass through, so repeat the exposure what actually makes it happen. Ammunition does not necessarily expire with age, but the power of the propellant may decrease over time. This can cause your ammo to get stuck in the barrel if you fire your gun with it. Keeping ammunition safe from moisture and degradation will help maintain its viability for firing.