well, the "gas" the nice people at the car shop put in your AC is called Freon, a chemical in which, when in a refrigerant state, is a gas... now all gasses will evaporate or at least diffuse within the air, so yes, one you have your AC recharged with the freon it will blow cold again until you need to recharge it, now depending on how much youll use it, in the summer it should last around 6-7 months

Every car AC seems to need to be regassed every few years. Not sure why, as home refrigerators don't....FREON used to be used, but it's a CFC so was phased out (see Montreal Protocol)

ANY gas under pressure will eventually leach out thru a fiber or rubber seal. Ever opened a flat bottle of pop?How long's it been since your A/C was serviced? 3-5 years between recharge is about average.

the refrigerant "gas" can leak out slowly over time, if it leaks faster then you have a major leak which you must get repaired.

looks like the lady didn't know what she was talking about. no, the refrigerant in your car will not evaporate or disappear in the system if it is sealed. main reason you would need to recharge your ac system is due to a leak, so have it checked out.

Explain in more detail please.

My cars AC isnt blowing out cold air anymore and the lady at the service centre said that the gas can evaporate eventually and it will need re gassing

Is this true? I never thought the gas could ever just disappear?