Engineer1911 nailed it.
Powder stored in sealed containers has been found to be good 50 years and longer. Ammo even longer.
For even the most anal retentive person, long term storage, and I mean REALLY long term storage, is done well enough by just storing it in authentic military ammo cans. They are air tight with rubber seals around the lid, and that is more than is needed.
What you want to look for is what could be the wrong way to store ammo, not the best way.
If you avoid storing it in hot, wet or humid areas, then you are good to go.
Even a garage in a Southern state that gets normally humid and hot won't hurt ammo in the long term. Just don't put it where it can get rained on or in an open shed somewhere.
My own long term storage is done for convenience rather than protection, and what I do is to save larger plastic containers with screw on lids, like this or similar: [ame="http://www.amazon.com/Fresh-Dolce-Classic-Italian-Biscotti/dp/B00BI48PRW"]Amazon.com: Fresh La Dolce Vita Classic Italian Biscotti 40 Oz of JAR@@AMEPARAM@@http://ecx.images-amazon.com/images/I/51IJWtUjO0L.@@AMEPARAM@@51IJWtUjO0L[/ame]
(I happen to like Biscotti in the morning

).
I can fill one of those up with around 600 rounds of .223 or 1500 rounds of 9mm, screw on the cap snuggly, and put it away in a closet for future use... maybe LONG future use. Of course, with the ammo, I drop in a note to myself detailing the load information for that batch.
One consideration I do give to the ammo I store long term is that I don't handle it with my bare hands.
I wear gloves during all processes at the point of tumbling and going forward anyway, so I also wear gloves while handling my loaded ammo for storage in order to avoid body oils from causing long term tarnishing of the brass while in storage.
Oh jeez, maybe
I'm the retentive one?
