its been quite a while for me since I visited an A/C shop - surely most of them in Karachi are thieves - when I redo an A/C system I take care of proper sealing, heat transfer, vacuuming, all switches/sensors working and using proper oil and refrigerant gas. Those car A/Cs work for about 6 or 7 years, some are still working with no issues - Toyota 1JZ Cressida, MB, Toyota corolla, Daewoo Racer
Its how much effort you put in it, sometimes A/C problems are masked by other problems and our mechanics in Pakistan love to leech money for no good reason. A regular term in the past was - denso bekaar compressor hai - sanden lagay ga - itnay ka hai - WHY, They made a hefty profit on selling it.