It depends on the car. I've got a 2002 VAG car that I've owned from new and the aircon has never been re-gassed and it's freezing cold still. Our other VAG car from 2007 needs it (at least) every other year otherwise it just gets really poor. The guy I use to regas says it's to do with the quality of the pipes used by VAG in later cars (or at least cars around the era of our 2007 car anyway), which were thinner, or leaked (or whatever), whereas the old pipes they used to use retained the gas better. No idea at all whether that's true, but I've no reason to doubt him.
