The good news is that cake-mix cakes are good keepers. If wrapped properly they will last up to a week (if you haven't gobbled them up by then) at room temperature or in the refrigerator, and they will stay fresh up to 6 months in a chest freezer.
The bad news is that many people don't wrap cakes in the right material, so they compromise the flavor of the cake. The refrigerator can rob a poorly wrapped cake of moisture. The freezer can rob a poorly wrapped cake of moisture and flavor. There are ways to wrap and store cakes successfully. But before you wrap, remember that it's easier to wrap a cake if the frosting is hard. That's not a problem for caramel frosting, which hardens as it cools, but it is a problem for the buttercream and cream cheese frostings that are soft to the touch. Chill them uncovered at least 20 minutes before covering so that the packaging material doesn't pull the frosting off the cake. Allow a cake to cool completely before freezing it.
If the frozen cake has a creamy frosting or a filling containing eggs, then it must be thawed in the refrigerator from 8 hours to overnight. Otherwise, you can thaw a cake on the counter overnight. If the cake layers are frozen unfrosted, and you want to thaw them to frost, follow a procedure recommended by Pillsbury's Sandy Nieman: Thaw them three-quarters of the way covered, then uncover and complete the thawing. This allows the layers to dry out so the frosting will adhere to them.