Unless you have a leak should not need to add Freon. Freon doesn't need to be changed a certified technician can check for leaks adding oil or freon as needed.
It really spends on the car. Some may never need it while most need to be topped off every few years. Just take it to a qualified shop and they can tell you how much needs added.
Todays cars operate on a closed system which means they shouldnt require more freon for many years. If your car is requiring freon be sure to have a mechanic check for leaks first before recharging.
An air-conditioning system should not need freon unless the air-conditioning system is not working as designed due to a refrigerate loss. There is no set time when freon needs to be added. You only need to have it added when it leaks.
Really it shouldn't need refrigerant(r34 not freon) for a very long time. If it need it less then 5-10 years then your system probably has a leak in it. I would have ac technician have a look at it and check.
You should only need to put freon in your car every ten years or so. It could need it more frequent if you have a leak in the system and that can be pricey.
A\c dont consume freon as car consumes oil,so under normal conditions freon would never need filling again. Low freon level indicates a leak somewhere which should be repaired before filling again.
