A/C problem in Chevrolet Fortester. What to do?

HI, i have chevrolet forester and i love it but from two weks ago the AC of my car had start giving some problem. After a complete time period of 9-10 months, i finally decided to take my lovely car to the workshop. First time it didn't get fully repaired and now it has been 3-4 times at workshop and didn't get the solution for my car issues.

Problem with the AC is the AC gas seem to be leaking. Each time, they have refilled the gas saying they are not able to diagnose any leak. After every refill, it seem to work well for some time (2-3 months) and the leaking happens all of a sudden rather then a gradual drop in the cooling functioning.

I don't know what to do. Need to here some useful tips from an expert please help.