How long will shells last
I agree with some aspects of what someone posted earlier.
It is NECESSARY to inspect your cartridges at regular intervals within the firearm, and your stock supply (unless the stock is placed in a dry, closed place.
Some of the problems encountered, especially with steel heads is corrosion, also the primer seal and ring around it can lose its integrity due to rust, which in the long run due to moisture ingress can make the reliability of your round questionable.
And trust me, you want to make sure that thing goes off when you need it rather than realise, Hmmm this thing doesn't seem to work, when in a sticky situation.
Prevention in this case is better than cure.
(reminds me of a case when cocking the slide of a semi-auto, the slide didn't go forward fully and the trigger wouldn't go off. I was thinking...if I had to shoot someone to save myself, I would have been a goner..(and yes I know it's a shotgun forum..)