Depends on for how long they sit before and after being used, and at what temp the storage area is at, sitting for long periods at cool to cold temps will harden them back up...keeping them in a warm to hot storage area will soften even more, best to try to figure out a place to store them that is somewhere in the middle, not too hot, not too cold...I prep with softener prep to get the duro reading where I want, then use no softening/high bite creating prep weekly, duro on tires stays very constant...if I see the duro maybe going up a little for any number of reasons, then hit them with a couple coats of softener to get them back down...