"How long can ammo be stored before the lube becomes questionable?"
I don't know yet. The oldest bullets I still have a little of was cast/sized/lubed in '65 and I know it's still fine so I doubt anything younger would cause a problem. The oldest ammo I have are a few remaining rounds of WW2 .30-06 but last I fired any of it, all was well; I might wonder about WW1 surplus ammo tho. ??