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There is an expected leakage rate for the A/C components. The components are rated in g/cm^2/year, but for hoses that is often simplified to grams per meter of hose. Some barrier hoses leak as much as 400 grams per meter per year while at 90C -- a typical temperature/pressure for hoses near an operating engine. Of course you don't run your car 24 hours a day, but it still points out that some minor permeation of refrigerant through the hoses is normal.
There is somewhat higher rate of leakage when the A/C is operating due to the higher refrigerant temperature and an increased lossage rate past the carbon face seal of the compressor drive shaft.
Occasional use of the A/C system is beneficial because the circulating oil mist redeposits an oil film on the system, helping reduce permeation. It's especially important for the compressor shaft seal.
You can expect to lose 2-6 ounces of refrigerant per year. As long as there is some liquid refrigerant remaining, the non-operating pressure is dependent on temperature not the fill percentage. A rate such as "10% a year" is definitely wrong, and a precision such as "10.7% per year" is absurdly wrong.
You can expect to need a can of refrigerant every few years in a hot climate with lots of A/C use, to once a decade in a cold climate with little use. If you are using more, there is a leak somewhere. A shop might use a "sniffer" to track down a moderate rate leak. You can add a UV dye to the system with the next top-off, and use a $5 UV LED flashlight to track down leaks, even ones too slow for a sniffer to detect.
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