the guy with the 1993 and 1997 is on the right track but you just dont charge 3x a year unless you need it.apparently on his volvos,he needs it because he has major leaks.

you dont charge unless you need it and you only need it if it is not cooling properly.get a cheap accurate thermometer and check the outlet air temps when the a/c is pumping out the max.the air temps should be between 40-50F.if outside this range,then you have problems and probably needs charging or if too cold it may need discharging.

auto a/c are prone to leaks because of the heavy vibrations and shocks they get by just driving the car in and around some of these potholes.these systems are not like your home a/c that just sits there and does its job without any abuse.

i agree with the other guy that it is not rocket science and if your volvo needs recharging then a kit is easy to use.just be sure you find the "low" side and not the "high" side.but i think by now they have different size connectors that make it foolproof.

if you want to be a pro then invest in some gages so you dont overcharge or undercharge the system.i wish i would have bought mine 30yrs ago.

good luck.
a bsme w/hvac experience