Not that I personally know a lot about these things but I always remember a guy on the radio (a car A/C specialist) talking about re-gassing being the biggest con job in the industry, because as Spartacus is saying why would you need a re-gas if you don't have a leak? And if they are just re-gassing without fixing the leak it's just a waste of time and money.
The A/C system is a closed system, there's no-where for the gas to go without there being a leak and ask yourself, how many times do you need to re-gas your fridge?
One very big difference between fridges and vehicle Ac systems.
Vehicles have the compressor driven by an external belt drive. This means that there is a shaft, with a seal on it, that separates the inside from the outside and has to contain the gas.
Fridge compressors, being electrically driven, are completely sealed units (Apart from where the wire goes in. Something that is rather easier to seal than a rotating shaft.)
Another massive difference which is far more important to consider than seals, is that your fridge does not move around over potholes and kerbs and, and get shaken around violently (in comparison) almost every time it is used.
It is the vibration on pipes joints etc that causes minuscule leaks almost constantly on mobile air conditioning systems.
Any kind of fixed installment of air conditioning or refrigeration systems, such as fridges, freezers, office A/C, shop chiller units etc are not allowed to loose any refrigerant gas at all over time, I have seen systems in place for 20+ years and they still have exactly the same amount of gas, to the gram, as when first installed.
However (and I cannot of the top of my head remember the exact figures but I think it's in the region of 10 to 15%) mobile A/C systems such as on vehicles are allowed, by regulations, to loose x percent of their gas each year without it being a concern, because the powers that be, know such mobile systems are subject to huge amounts of vibration fixed systems are not exposed to.
So yes you might be lucky and have a car that looses nothing over a few years, or you may have one that needs topping up every couple of years or so, but unless a major leak is found that looses it all in a matter of weeks, having your system topped up every couple of years or so is normal and not a need for concern.