As far as shelf life of shells go, I once had a pleasent morning a few weeks back with a few boxes of about 1970's vintage paper shells that my grnadfather had. Its hard to describe, but paper cartridges do have a very pleasent smell
to them after you have fired them.
I would definatly change the shells in the mag of your shotgun every two months or so. In (fortunatly) rare cases, the pressure of the magazine spring can "mash" a plastic shotshell creating a jam that you have to see to belive. I think this is normally caused by el cheapo magazine extensions, but why take the risk. Dont throw the shells away- put them in a box and use them for target practice one day.