It varies a lot depending upon a number of factors. Exceptionally I have serviced AC systems on cars that were still working - just - after ten years without being recharged. In 2005 I saw a Mercedes that was 22 years old and the AC was still just about working. It is almost 100% certain that it had never been recharged in its entire life.
A more usual time for the first recharge would be around four years. The old R12 systems would usually last slightly longer than R134a systems as the R12 molecule is larger and the type of oil used with R12 provided a better seal on the compressor shaft I believe. At four years old the AC system would still be working reasonably effectively even with a good proportion of the original charge missing but by this time for slightly complex reasons some moisture would have been able to penetrate into the system. Some of this moisture would then mix with the chemicals making up the refrigerant, creating acids within the system which will do no good at all and will start to corrode the system from the inside.
A good rule of thumb would be to recharge a new car after no more than four years and then to recharge at three-year intervals. If you have a fairly recent car it may need recharging slightly more frequently now. Newer developments in AC technology are resulting in systems which are able to cool well with much less refrigerant than just a few years ago. In addition to this all new cars from 2008 are obliged by law to have very reduced natural losses. In spite of this it seems that if a car has only a small amount of refrigerant in the system, the loss of perhaps only 50 or 100 grams, a fairly small percentage, seems to have a serious effect on the performance of the cooling. As a result I am having to revise my thoughts - three year intervals may be fine for any car pre-2000 but for some cars built after 2000 it may be necessary to recharge every two years to keep the AC sufficiently effective especially if driving abroad.
Most problems occur when the system is struggling because it has insufficient gas to do it's job properly, and some of the results can be expensive to put right. It's usually much cheaper to maintain it regularly every three years than to wait for a breakdown and possibly an expensive repair. Particularly do not leave a car with a low charge over the winter months - this is when the air contains most moisture in it and it is likely to enter the system. This is particularly important with Variable Displacement Compressors. These run all the time once they are switched on and during the colder winter months are compressing very little refrigerant. If in addition there is a low refrigerant level there may be insufficient gas being moved to keep enough lubricant in the compressor. Each early spring we have to replace quite a number of variable displacement compressors that have seized or partially seized and broken up during the winter. Get it sorted when you first notice it and you will still have a system next spring - leave it and you take a big risk.
In the last six or eight years a new clutchless type of compressor has emerged and is being fitted to a wide selection of cars. This type is constantly running and electronically controlled but cannot be turned completely off, in fact the OFF button will leave the compressor turning all the time but compressing only 2 or 3% so in effect virtually OFF. In theory these systems cannot be left safely without a refrigerant charge in them, so repairs will need to made when the fault occurs and not left until it is convenient to do so, unless you are prepared to take the risk of having to replace the compressor as well.