For most hunters and sport shooters who use their firearms regularly, you don’t have to give too much thought to your ammo aside from keeping it dry and safely secured when you’re not using it. Beyond that, you’re cycling through your inventory pretty consistently with few issues. Modern, factory-made ammo is very reliable and will last for a long time under the right conditions, however there are instances where you could be pushing your luck.
Two related interactions I had got me thinking. One buddy (who isn’t well-versed with guns) had ‘inherited’ a decent 
handgun from his Father-in-law. He decides he’s now going to conceal-carry since his neighborhood is kind of rough and gets himself set-up for it. Now he’s had this gun for a while and I ask him if he goes to the range to practice. He says he’s never fired it. I naturally ask if he knows if it will fire. How old is the ammo? Are the magazines in good shape? He’s got no clue.
I was borrowing a tool from an older neighbor’s garage and noticed a box of cartridges 
in the toolbox. These things were dirty, looked ancient and did not appear to be in good shape. Used for ‘critter control’ only apparently. Where we lived at the time, the temperature can range from up to 90 degrees in the summer with high humidity down to well below freezing in the winter. Now I’m no expert, but neither of these scenarios sounds like an ideal way to store something as volatile as live ammunition.
Could these rounds still fire? Maybe – but why take a chance – especially if you were counting on your firearm for your personal safety.
So does ammo actually have a defined ‘shelf life’?
The answer is not as clear as you may think. There are a number of factors that can adversely affect how long ammo will retain its full effectiveness. Luckily, you can help to offset most of these with a little common sense.
Most all manufacturers state that ammunition will last for 10 years (to be conservative), but the truth is that it will last much longer if stored properly. Like most things in life that can degrade over time, the biggest culprits are heat, moisture and sunlight. These elements can play a part in causing corrosion or breaking down the chemical structure of your powder. Either way, the safety and performance of your munitions can be affected negatively.
For the most part, all you need to do is to keep your inventory in a dark and dry environment with a moderate temperature (think wine cellar) and you should be good for a VERY long time. If you have high humidity and keep more than the average 
number of cartridges on-hand, you could consider using airtight ammo cans (https://www.k-var.com/gun-guard-ammo-can-od-green-6pk) and supplement with those little anti-moisture packs. A de-humidifier is another option. You should also avoid keeping ammo anywhere near oils or solvents as the proximity can affect the chemistry.
For those that do keep a large amount of ammo stored (for whatever reason), it’s normally good practice to keep track of your stocks by date and utilize the first in-first out (FIFO) approach to ensure a good rotation. Kind of like the dairy section at the supermarket. You’ll always want fresh product.
As far as concealed-carry goes – if you don’t practice or maintain your firearm regularly (which you should), then you might 
want to get into the habit of hitting the range every now and then or at least swap out your ammo a few times per year just to be safe.
What about magazines?
Like ammo they CAN last for a long time, but why take the chance, especially if you depend of your weapon for your safety or the safety of others.
Magazines are very simple devices and it’s really just the springs that you may have issues with (although polymer followers CAN break). Some combination of poor quality manufacturing, corrosion or dirt and debris in your mags can cause them to 
wear-out prematurely or not operate as they should. The chances of a magazine failing due to overuse are slim, but it’s easy enough to swap-out parts or replace the whole thing if necessary.
The other thing to consider is how long you leave magazines fully loaded. Springs can lose tension over time, so it’s always a good plan to unload them occasionally to allow them to ‘rest’. This is also a great chance to break them down for inspection and cleaning. Having a second set of magazines that you can cycle-in should be standard practice.
How often should you undertake this rotation? You’ll likely get different answers depending on who you talk to, but the general consensus is to follow normal law enforcement standards and do it every 90 days.
The longevity of your ammo and your magazines is all relative and pretty much based on how you handle and store them. There’s a better-than-average chance that you’ll never have an issue, but it’s always a good plan to keep the odds tipped in your favor.
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I am not an engineer so I do not claim to have the final word on this subject, but I have read many articles over the years on the subject of springs “losing their tension“ from being compressed for too long. I have read many people who claim to be engineers and who state that it is not continual compression which weakens springs but repeated tension and compression and cycling of the spring. The above article states that springs can lose their tension from being compressed for a long time, but I have read many times that such is not the case. Again, I do not claim to be the final authority on this, but I have seen the opposite information quite often.
Hey – thanks for the comment. I had read something along those lines as well. Typically, magazine springs are designed to work perfectly well for thousands of cycles of expansion and contraction. The idea of stripping them down and checking occasionally is more of a safety measure and way to avoid any ‘potential’ issues. Like anything mechanical, if you don’t use it you lose it.