People don't realise that the aircon system needs regular maintenence. You're supposed to get it serviced annually, but nobody does.
The gas lasts between 3 to 6 years depending on use and leaks. Ultimately the gas leaks in all systems (even without a fault), it will physically leak through the actual rubber on the pipework. If the system was constructed like a fridge, with metal pipework, then it would last forever.
A regas will cost between £65-£100 depending where you go. If the vehicle is out of warranty I would take it to an aircon spercialist (especially if you've got a fault or suspected leak). Audi seem to know very little about aircon systems, although that won't stop them charging £100 for a regas.