How long do your quick oats/oatmeal last? Flaked barley will last just as long.
I've seen oatmeal "go bad" exactly once (where there wasn't moisture problems). It was some quick oats in a non-sealed bag that was probably on a shelf in a dank storage room for at least 2, maybe 3 years.
In general, unmilled (and flaked sort of counts as unmilled) grain will last a year or two easily if stored relatively dryly, away from pests and vermin.