All a/c systems (residential, car etc..) are closed loop systems. Nothing gets in and nothing gets out unless there is a leak somewhere. You should not have to "service" the A/C ever unless there is a leak of some sort. If there is a leak in the system and you keep putting freon in it eventually the compressor will stop working because of all the air that gets sucked in. I would find the leak with a refrigerant detector or take it somewhere and have it looked at. The other problem associated with the loss of freon is the loss of oil for the compressor -the two go hand and hand.
