I'd do it once a year max. myself. It may not ever really be necessary, depending on certain things like whether the car burns oil (and how much) as well as how often, and how, you drive it. There's going to be some buildup over time - as the car sits, fumes from the PCV inlet and outlet can reach the throttle body, oil can even run out of the pcv inlet into the intake tube, as well as from the combustion chamber - but usually not too much. Even after 15 years of driving the car, it's going to be noticeable but may not actually have a significant effect on the idle, etc.