Car air conditioning systems really need regular servicing due to the natural porosity of the hoses and components of the air conditioning systems in modern vehicles. The refrigerant gas will leak slowly out of the A/C system at a rate of 15-20% per year, thereby making your system gradually less efficient until it finally stops working all together.
If you let your A/C system run out of refrigerant and then you don't use the system for several months the chances are that internal component seals will dry out and you will have leaks in your system when it is eventually re-gassed. This could be expensive!
Always ensure that your aircon system is in working order by running it on full cold at least once a week for a few minutes, even in the winter at the risk of frostbite! It takes at least one hour to carry out a correct service and re-gas of car aircon systems.