Fimo stays pretty good but if it does get dry you can bring it back by adding Sculpey Mold Maker to it.. I have used the mold maker to add to Fimo classic to soften it up a little when I needed, it makes it more like the Fimo Soft. I little of the mold maker goes a long way though.
I have Fimo clay from october and it's still good I have never actually had polymer clay dry out on me. I do have keep it in a plastic tackle box.