When your air conditioning is blowing warm air (or even just-not-really-all-that-cold-air) it could be time for an AC service.
The stuff that makes your AC cold is a gas. When the AC stops working, it's often because the system has lost some of the gas and, causing the pressure to become low in the system. During an AC service, that pressure gets recharged to the level it should be at, making the system able to cool the air coming into your car again.
When we do an AC service, we also always put a dye in the system that makes it easier to track down potential leaks in the future. (Since the refrigerant is a gas, it can be a lot harder to pinpoint a leak without the dye to help)
There's no set time or interval that you need to have an AC service done. It's only needed when it stops blowing cold air.