I bought a large quantity of new primed Winchester .25-06 brass about thirty five years ago at an auction. These were in boxes of twenty pieces. Some here may remember when the big companies sold new brass primed or unprimed.

These have been stored in a garage in Texas. I still have several boxes of this brass. All the primers have worked as they should. It matters not if the primers or in their boxes or in primed brass. As long as you're not storing them unprotected in your backyard under a shade tree, primers will keep for many decades.