Modern aircon systems aren't totally gas tight, and they don't hold a large amount of refrigerant, so it's best to get a periodic service. This should include a gas top up, and addition of a dye to the system to check for leaks. This tends to be best done yearly, due to the small gas reservoir in the Golf. My Subaru used to easily go two years between services.
But it doesn't need to be your dealer, any specialist can do this. Kwik Fit were offering a 'colder zircon or your money back' on aircon services not long ago.
