What? all systems leak -rubbish. My 1965 T-Bird was last charged in 1995 - still ice cold today - 12years later (gotta luv R-12, till you have to pay for it). Same with my 94' E-150 with R-22 I have added an oil shot to the compressor but never added gas, still stupid ice the windows cold.
The gas never actually goes bad, it either leaks out or gets contaminated - wrong oil, too much water in the system. The newer systems do seem to leak more than the old R-12 I believe the molecules are much smaller and can pass through seals more.
The regs in my area are so tight that if a system leaks there are 2 options repair the leak or evacuate the system. Some mechanics are pretty cool, but if they get caught charging a system that leaks they can loose their license to service A/C.
Anyway, find a tech with a trace wand or similar device. If it leaks this tool should be able to locate it.