How often should central air units be recharged?

Ideally, never. If you periodically need to be recharged, note the amount of refrigerant you take on the techs service ticket and also the elapsed time since the last recharge and that is the size of your leak, ie 5 lbs every 6 months. Refrigerant is not a fuel, it does its work by continuously recirculating. Much like the oil in your car except there are no products of combustion involved so it does not get dirty or break down and therefore does not need to be changed unless there is a compressor failure. The larger the leak the easier it should be to find, and depending on where it is fix. Usually the hard part is finding it. If your system is older and given the hourly rates you have paid and will continue to pay for recharging, refrigerant costs, and what you may pay to find and fix a troublesome leak, replacement / upgrade can be an option worth considering.