We recommend washing your vehicle at least every two weeks. If you commute over 150 miles a week; or your vehicle is constantly exposed to adverse elements(bird droppings, trees, industrial fallout) you might want to wash it more often.
Don't forget road grime, brake dust, acid rain, smog settlement, and other pollutants have a harmfully effect on your vehicle's paint.
(* + higher price would apply to dog hair, sand and cars that require more care. )