Once or twice a year whether it needs it or not.
Usually when it shows dirt or pollen, etc. Having a silver car helps. My bugaboo is a clean windshield. That I do several times a week. Then a quick wipe down of the dash and center console. Now that I'm retired and put less than 5k miles/year on the car, it doesn't really get too dirty or bug covered as often as it did when I was commuting 2 hours a day.