After the clay has dried to the right consistency and compressed or wedged, it is ready to use. If you don't want to use it right away, store it in heavy-duty plastic bags. Freezer bags are good for small quantities.
Note: Most plastic is not truly air tight; air does move through the plastic, just slowly. Some plastics are much more air permeable than others.
If it will be several days before you use the clay, place the plastic bags of clay into an air tight container, such as a plastic tub with a tightly fitting lid. Clay can be kept in a workable state indefinitely, if it is kept in an air-tight container with little to no air in the container with it. Clay is not like food. It can't "spoil". (It's actually already decomposed -- decomposed rock.) What it can do is dry out.
