Never, at least not in the way you think of your oil being changed. Your car's AC system is a sealed unit. No new Freon needs to be added unless there's a refrigerant leak. Over time as your vehicle ages small cracks may form in your AC system parts such as the compressor, condenser, evaporator, receiver-dryer, expansion tube and connecting hoses allowing Freon to escape. If this occurs, you need to replace the failing part and have your Freon recharged.