The quick answer is “never.” This is because most modern air conditioners are delivered with the precise amount of refrigerant that it will require for the rest of its life.
In fact, unless there is a refrigerant leak, a central air conditioner should never require more refrigerant. You'll need a professional to identify and fix a refrigerant leak if you suspect one. Refrigerant is a poisonous chemical that can cause a variety of problems, including breathing difficulties and even death. So, unless you're a trained refrigerant handler, leave a refrigerant leak to the pros.