NEVER, if the unit is properly charged by the Thermocharging or superheat method and has no leaks. There are two reasons refrigerant is added to an air conditioning system. One, is because most service companies do not train their service technicians to use the Thermocharging or superheat technique and their service technicians erroneously add and subtract refrigerant as they see fit. The second reason is because the A/C system is low on refrigerant due to a leak somewhere. Refrigerant leaks should be found and repaired as soon as possible. An A/C system low on refrigerant will run much longer than necessary, make it hard to cool your house, and increase your electric bill substantially. Plus, the compressor will become damaged and will prematurely fail resulting in a very expensive repair or replacement bill.