Its just a system similar to a domestic fridge it is filled with gas to a certain pressure (recommended by the manufacturer of the car) normaly R1234a in new cars although there are different types being used the fitter uses a set of gauges so he gets the correct pressure the gas comes in a cannister.Why would you want to extract gas it is very expensive and you are not supposed to let it out into the air.

The optimum vacuum pressure is -1 bar..The vacuum programme is done after the existing gas (if any) is removed from the system.The length of time required for the vauum process depends on the type of work that is being done to the car.A basic regas of the system where it is currently working,a time of between 15 and 30 mins would be adequate.A more complex job where the system has been unused or open to the elements,or a contaminated system or a car with double a/c (front and rear) would need much longer.hth

ideally the vacuum would be held for about 30 mins unless circumstances suggest otherwise (see above answers). Doing so not only proves system integrity, but the vacuum will 'boil' off any moisture in the system.

Bear in mind that this 30 mins is garage time that must be paid for. A cheap "regass" will probably just be a top up. Doing it properly costs extra!!!