Use unsweetened Kool-Aid packets — the condensed milk handles all the sweetness. Stir condensed milk and Kool-Aid first until color is even, then fold in Cool Whip gently to keep the filling light. Minimum 4 hours to set; overnight is better. Shortcut: freeze for 2 hours instead and pull out 15 minutes before serving. Store covered in the refrigerator up to 4 days.