How to Store Polymer Clay, Millefiori Canes or Scrap clay?
Polymer clay can be stored for months or years. Exposure to air will not cure or hurt it, but it's best to keep it wrapped for protection from dust, animal hair, sunlight and lint. polymer clay does not contain water that can evaporate like earthen clay. The main danger to polymer clay is from excessive heat or ultraviolet light; these can partially cure the clay, making it unusable.
Your clay will last long if you keep it away from sunlight and heat.
If you are buying clay in a store and have the opportunity to test it, always check it for hardness, since occasionally clay is left in the sun on supplier car or otherwise mishandled in a way that causes it to harden prematurely. (Some clay brands are harder than others and only slightly pressed with your fingers - such as Fimo Classic or Kato.)
With all that said, don't be afraid to use old polymer clay. I'm using 2006 or 2007 packaged of polymer clay and they are still good! i wouldn't give them to children to work with, but they can be conditioned using body warmth, mixing old polymer clay with fresh polymer clay, adding mineral oil or clay softener and using a pasta machine. When you store you polymer clay correctly you can work with it for many years!
Storing polymer clay and millefiori canes - You can keep a finished (un-cured) piece as long as you want before curing it. As always, make sure not to let dust get on the pieces, and make sure that un-cured canes aren't stored touching each other or they will start to bond together and won't be separable without damage and if they have a different background color they will stick to each other and stain each other. I am storing my un-cured millefiori canes in tin boxes (usually tin cookie boxes) or plastic boxes divided into colors (each color of canes has their own box).
You can also store canes in 35mm film canisters with some wax paper for easy removal. They are air tight, and water proof. I also use them for seed beads, and small silver findings I do not want to tarnish. Any film developer will have them, and most likely love to give them to you.
Scrap clay can be easily stored in plastic ziplock bags - either condition and roll to thick setting and than placed in the bags, or as chunks.
Another friendly solution for your canes and new packs of clay are plastic multi-drawer compartments that you find at hardware stores for nails, screws and such. The small drawers are the perfect size for clay blocks, and the larger drawers are great for keeping jewelry findings, beads other items.You can add wax paper to keep the clay from reacting to the plastic in the drawers.
Be careful about storing polymer clay in plastic containers or plastic wrap. Some types of plastic can be used with polymer clay, but others react with the plasticizer in the clay, and the clay eventually will begin to bond with the container. Heavy duty plastic boxes are usually good for storing raw polymer clay in it but you can always place wax paper on it's bottom to make sure.