The best indicator that you need to recharge the refrigerant is when the system begins to cool less than it used to, but before it stops cooling completely. When your system doesn't blow as cold as it once did, you need to have it inspected.

How do I know when my AC needs to be recharged?

In the simplest form, you'll know when you need an AC recharge if your AC starts pumping warm air instead of cold. There is a very limited amount of refrigerant in the system, and when even small amount leaks, the system can no longer function properly, delivering lukewarm air instead of cold air.

How often does AC need to be recharged?

An AC doesn't “use up” refrigerant. So you should never need to recharge your air conditioner with more Freon—unless there's a leak.