Before I answer this question you should know that your air conditioning unit is a closed and sealed system. The refrigerant that is in your AC system recycles itself through all of the various cycles that it goes through. The refrigerant should never run out and you should never have to refill your system unless there is something else wrong with your unit.
For example, if you have a leak somewhere in your sealed system than the refrigerant will slowly dissipate. If you add more refrigerant to your system you are not solving the problem. That refrigerant you just added will now be leaking out of that same hole. What you should do in this instance is to identify the leak or problem area, fix it, and THEN add refrigerant back to your system.
If you have a contractor saying that they need to refill your refrigerant and they do not mention a leak or other problem with the system you should be very skeptical. Are they fixing the problem with your unit, or are they gouging you on a refrigerant refill and then a week or so later they will be back out there when all of the refrigerant they put in has leaked out.