Base Mix: In a large mixing bowl, combine the sweetened condensed milk and the thawed whipped topping. Whisk or stir until the mixture is completely smooth and no white lumps remain.
Add Milk: Gradually pour in the half-gallon of chocolate milk. Stir well until the mixture is uniform in color and consistency.
Transfer: Pour the liquid into a freezer-safe container (a large 9x13 pan or a plastic tub works best for even freezing).
The "Slush" Step: Place the container in the freezer. Crucial: Every hour, take the container out and give it a vigorous stir with a whisk or fork. This breaks up ice crystals and incorporates air, creating that signature Frosty texture.
Serve: Once it reaches a thick, soft-serve-like slush consistency (usually after 4–6 hours), scoop it into glasses and enjoy immediately.