As with so many other things, the real enemy is moisture here. While it is not as if your chalk is going to be unusable if left in the air for several hours, you will want to store the chalk you are not using correctly. And, because most chalk bags will not seal 100 percent of the way, it may not be wise to leave the chalk inside.
Instead, find an airtight container or bag to store the chalk you are not using in. This will prolong its effectiveness.