In my experience, with an older car, they suck out the old oil and gas, fill it up, find a few leaks, empty it and fix the leaks they've found, fill it up and give the car back with a bill. After a few weeks or months, it escapes through another few small leaks they didn't find last time, and you have it done again. Then repeat steps one and two once a year.
Car systems are made of several pieces connected together with metal and rubber piping' each of these joints can leak at a seal. Once it has been opened up it will never be as good as the factory build. One of the reasons domestic fridges and freezers last so long is because all the parts are soldered together and there are no serviceable joints. Even the motor and compressor are in a sealed metal housing.
My bro-in-law does commercial and yacht air con and refrigeration plant (in Australia), he won't do cars because they have a high failure and return rate, often from a different fault which occurred due to it being the same age as the other bit that he repaired last time.
You can't do it yourself as you have to have it thoroughly sucked out with a vacuum pump; not just because the gas is environmentally damaging, but also because damp gets into the pipes and will damage your a/c system when it heats up.
