Polymer clay, being non-volatile, can be stored for months or years. Exposure to air will not hurt it, although it's best to keep it wrapped for protection from dust and lint. Unlike true earth clay, polymer clay does not contain water that can evaporate. The main danger to polymer clay is from excessive heat or ultraviolet light; these can partially fire the clay, making it unusable. Over time, the plasticizer that keeps the clay soft can also leach out, leaving hard, crumbly clay.

A common rule of thumb is that clay can be kept at least a year, sometimes for several years. Your clay will last longer if you keep it away from sunlight and heat.

The rule of thumb here is that if you can still squidge the clay between your fingers - if it yields to pressure - then it can be worked. Borderline clay can often be rescued by adding softer clay or other substances to make it easier to condition.

If you're 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 loading docks or otherwise mishandled in a way that causes it to harden prematurely. (Remember that some clay brands are harder than others and will normally yield only slightly to the pressure of your fingers.)

In general, you can keep a finished piece as long as you want before firing it. As always, make sure not to let dust get on the pieces, and make sure that unfired canes aren't stored touching each other or they will start to bond together and won't be separable without damage.

If you've conditioned a piece of clay and have then stored it for a while, you don't need to recondition it; it will retain the conditioned texture indefinitely. However, you may want to warm it before starting to work with it; warming makes the clay softer and easier to work with.

Since polymer clay is a plastic, it's very durable. The colors of polymer clay generally are stable in normal use, although a piece exposed to direct sunlight for years (such as a suncatcher or wind chime) will probably fade. There are artists who have pieces decades old that still look like new.

Thin pieces sometimes do break or chip. If your clay piece will be handled a lot or be in danger of bumps, and it will include protrusions or thin pieces, choose one of the stronger clay brands, and fire it for a longer time than usual for added strength.

You don't need to store polymer clay in airtight containers, since it contains no water to evaporate. However, it should be wrapped or covered to prevent dust and lint from getting on it, and kept away from heat and sunlight.

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. In particular, avoid putting polymer clay in polystyrene (recycling #6). Polyethyline (#2 or #4) and polystyrene (#5), however, seem to be compatible with polymer clay for storage.

For similar reasons, you shouldn't store different colors of clay without putting something between them, or store fired clay in contact with unfired clay. They'll eventually start to bond together.

You can wrap polymer clay in waxed paper before putting the clay into a storage box. The plasticizer does not react at all with waxed paper, although some plasticizer may leach into the paper over time. You can also store finished pieces (unfired) and unsliced canes this way.