Your air conditioning system is a closed loop system that only becomes low on freon if you have a leak in the refrigeration lines or coils. A normal operating system circulates freon from your outdoor condenser to the indoor evaporator coil through copper lines using a compressor. Freon does not get consumed by the normal operation of the system. The only way that a system ( if properly charged upon start-up) can  become low on freon is if it is losing the freon through a leak. Leaks can occur in coils or copper lines and can be caused by factors such... Continued