Royal will last a long time if tightly sealed. I keep mine in a tupperware tub and leave it out, it does not need to be refrigerated as there is nothing in it that will spoil. When you are ready to use it you may have to stir or possibly add a dab of water to get it back to the consistency you want.
Do as the others have suggested, but when you come to use it again, check the consistency, you may need to rebeat it a little if it has lost its stiffness, or add a little water if it has become too stiff. It is vital that no air can get to it, once it starts to dry out there is no turning back!
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