I would get the aircon serviced once a year,In a typical vehicle it should take about an hour, including replacing the pollen filter, disinfecting the interior air supply, and a full re-gas (refrigerant)and more importantly the compressors lubricating oil that is suspended in the gas.
This job is best done if the vehicle is in for scheduled work as the equipment used is fully automated, and can be left conected to the vehicle while the technician works on other tasks within the service.
Any less than a 50 minute cycle and its not a proper service,charges are going to vary, but ask the right questions, get the right answers,and then make your mind up..I would avoid quick fit and go to a reputable technician/specialist.
To keep the system reliable use it as much as possible,even in winter for de-misting, fuel economy is only effected on really hot/humid days when the system works harder to keep the interior cool. hth