Properly carried out an air con service is more than a regas.
The refrigerant in the system should be removed and measured for quantity. This lets the operator know how much is missing and is an indication of whether the system has a leak although there is no indication of when the system was last serviced.
The system is then recharged with the correct amount of refrigerant including a lubricant to keep the components healthy and is then checked for leaks by seeing if the system holds its pressure.
If there is a leak a dye can be injected to assist locating the source of the leak.
Ideally an aircon system should be serviced every two years although modern systems can probably go to three years.
Vic.