With modern technology and the quality of plugs available from the normal plug through to the expensive plugs ( platium) and the high voltage and low current through the plugs they will last over 100.000miles. The ECM continually keeps the engine in tune. So all that may need changing is the HT leads. There is an engine light in the car that will let you know when something is wrong and all you need is to have the fault codes read if you feel something is not right.. Some cars (V6 Mitsubishi) have platinum plugs in the rear bank as it is impossible to change them easily and they are not touched at the services and very seldom play up.