Good question ! I know there is a specific humidity level for cotton ... but could not tell you off hand what it is. I do know refolding is a good pratice. Fabrics can develope permanent creases if left folded too long with the same crease, how long that time frame might be.. I think there are variables , heat, humidity that would need to be calculated. I hope there is a good rule of thumb . ... but on the other hand .. refolding my stash.... now thats a project and then some.
I love your term "excavating" for digging through stash... have you uncovered many "fossils" in your digs ?!