avoid hellfrauds like the plague, they are cheap, but they buggered up my compressor a couple of years back by using the wrong pang oil (there are different types as well as amounts)
re gas every three years, and as mentioned above, its perfectly normal to loose gas over time in a car air con system.
if its climate control the compressor will be permanently running unless its in econ mode, straight air con is on or off, and normally comes on when you use de-misters, regardless of the button position.