To do the job properly you should do a pressure test with Nitrogen and a vacuum test, if either are not done you are not doing it correctly.
Not heard of the pin hole thing before but every day is a school day, it is usually seals drying out, a failing hose, joint or component.
Any car that can not hold its charge for at least 18 months has a leak that needs fixing, I have worked on cars that still have a good charge of refrigerant after 10 years and they still work.
If a recharge was several hundred £'s I suspect a lot more cars would be repaired or have no A/C.
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