It depends on how much you dive. If you dive 40 - 50 dives a year, we recommend that you service your regulator once every 24 months.
If you dive more than that, you should service it once every year.
Those that dive every day, should have it serviced at least once every 6 months or so.
Remember that your regulator is life support equipment, designed to keep you alive. If you don't maintain your regulator, it could cause a failure of the equipment.