How often does an air conditioner need to be recharged?

How often does an air conditioner need to be recharged?  The short answer is never!  If you need to add refrigerant to your air conditioner, it means that there's a leak in the closed loop, and once you're at a point where you're adding refrigerant to recharge, you're dealing with a ticking time bomb because as refrigerant comes out of the system, you're losing money, hurting the environment, and, for sure, if you don't stay on top of it, there will be times when the compressor is running without enough refrigerant, and that will fry it eventually.

In commercial and home units, 90% of leaks happen due to improper installation. So, pressure test the loop before releasing the refrigerant, and then make sure your vacuum consistently at the same negative pressure even when the vacuum is turned off.  Finally, after releasing the refrigerant, bubble test your SAE 45 (flared fitting) connections with dish soap and water to make sure that they aren't bubbling up.  You'll certainly see buttles if there's a leak in one of your connections.  

Meanwhile, car air conditioners seem to be more prone to leaks some years after they were (ostensibly) properly installed at the factory.  Why?  Heat, vibration, and O-rings.  The seals for r-134 low-pressure refrigerant are generally done with rubber (neoprene) O rings.  It's a matter of time before they wear out.  But, once again, if you're adding refrigerant to your car's air conditioning loop, eventually the compressor's going to see air, dirt, or lack of oil (from low refrigerant). 

Holes in closed loops aren't good and need to be repaired soner rather than later.