You can store it dry for as long as you want. I would think that trying to store it wet would cause the pieces to clump together badly when you try to get it out and spread it in your new viv. The finished clay product should not be handled when wet or it will clump together.
I just meant that I have a wet mix on hand. When I am ready I take out what I need, air dry it, press it through the screen and then bake it.