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 as: vibration, rubbing of copper against metal, or poor installation. Running your system while it is low on freon can cause damage to many of the working parts in your system including the compressor and can cause your energy bill to skyrocket as it loses capacity and tries to cool your home. Leaking freon also damages the ozone and environment. 

If you suspect that you have a freon leak you should call a local heating and air conditioning contractor to inspect your system and try to repair the leak. Most leaks can be found using electronic leak detectors, soap bubbles, or other leak detection devices. If the leak can be repaired then new freon can be added to your system and normal operation can be resumed. Sometimes the leak cannot be repaired and a part may need to be replaced to remedy the situation. Either way you should have a leaky system repaired to save both money on your energy bill and help protect our environment.