How hard your tub frosting gets and how quickly it gets there varies depending on the type you use. Whether your store-bought frosting is buttercream, cream cheese, royal, whipped or another type determines different characteristics. If you want a frosting that dries as hard as candy, royal icing is what you want. Most other kinds of icing don't get as hard and can be easily cut with a knife. Still, they can be hardened somewhat to help protect the integrity of your icing job and design for transport or before serving.
Cool your cake, brownies, cookies or other baked goods completely before frosting them with tub icing. Heat melts frosting, making decorating more difficult and preventing the icing from hardening.
Apply the coating of frosting thinly and evenly with a butter knife or offset spatula because large blobs or thick frosting will take much longer to harden. If you're making flowers or other decorations out of the icing, these will have to be thicker and will require more time to harden.
Set your cake or other baked goods out on the counter to air dry after frosting if the directions on your frosting package specify the product hardens that way. Make sure the cake's not in direct sunlight or fluorescent lighting, or the icing color may change. Heat and humidity will prevent frosting from hardening, so try to keep your kitchen cool and dry.
Refrigerate your cake or other baked goods to harden tub frosting faster. Use a cake tray with a cover to protect your icing and design.
You can cover your frosted cake or other baked goods with plastic wrap if you stick a few toothpicks in the top to keep the plastic wrap off the icing. This leaves some small holes in your frosting. Usually, tub frosting won't get so hard that you can't smooth it out over the holes before serving, though.
Most cakes and other baked goods also become slightly harder along with the frosting when refrigerated. Take them out at least 30 minutes before serving to allow them to warm up. As long as they're not out in a hot or humid environment, the frosting should stay hard.