Grant,
I see from your info you are from the Kansas City area (?). If so, where you live round bales should last 2-3 years left outside, because your climate is relatively dry. Of course, some of this depends on the size of the round bale. Here in the south, I've walked up to round bales that had been sitting on the ground for two years. The outside edge, about a foot or more, was rotted, but reaching my hand into the middle, I pulled out handfuls of good, nice smelling green hay. Getting them off the ground on old tires or pallets helps even more.
Unaka