The manufacturer's recommendation is an annual service. Some manufacturers have a lifetime guarantee that requires this.
If you don't use the reg much you can get away with less frequent services, provided you make sure the reg is properly looked after between dives. If you mistreat the reg then you should service it far more frequently.
After use the reg should be soaked in warm fresh water for 2-5 hours (more will not hurt - overnight is fine). The best way to do this is with the reg pressurised to stop water getting in. If you can't do that then at least be totally sure the reg is well sealed.
Rinse the reg under gentle running water and allow it to dry properly then store it in a warm dry place.
If you leave the reg wet with seawater in the bottom of a kit bag it will not last very long...
Chris