The general answer to the question of ammo shelf life is "indefinite".
From reading several threads on this, they all seem to agree that
- Moisture is detrimental to long-term storage - case corrosion
- Extremely high temperatures (120F+) can degrade the powder
IMO, indoor storage in original packaging is sufficient, without worrying about it further.
However, for really long-term storage, you might worry about things like cardboard boxes and dividers can have a slight acid content and attract moisture. Milsurp ammo cans are just about air-tight, so some repack ammo in ammo cans, with a dessicant.
I've read about a somewhat extreme practice of using a vacuum food sealer to repack ammo, and then store the results in ammo cans.
I guess it depends on whether you're looking at a 5 year horizon with life being somewhat normal, or a 50 year horizon with hurricanes and tornadoes thrown in...