The use of r12 in new vehicles, the ozone thinning gas was outlawed in Australia in 1994, VTs use r134a, which while being ozone friendly is not human or greenhouse friendly, it is about 1000 times more potent than co2 in IR redirection, though as far as I know its only the toxic effect on humans which make releasing it into the environment illegal.
Repairers should recover any r134a in the system and not regas systems that are leaking. However, the definition of leaking is the troublesome one, the systems are never completely leak proof with rubber hoses and many joins. If a system only loses its charge after 2 years or so, then it is more economical to recharge, rather than replace compressors etc, just have to hope that the repairer cant identify any leak at the time.
Does the refrigerant go off, no, just like in your fridge at home, it will work well forever if it doesnt lose any refrigerant.
Shop around for a good price, the price for recharges is high considering the $ value of the gas that goes in there is less than $15. There is only a small amount of labor, there is investment in the equipment, but $1000 is about all they need to setup for a good vac pump and set of gauges. The one issue that restricts profitability is that to properly evacuate the system, it should be on the vac pump for at least half an hour.