The physical service life for a spark plug can be thought of as the number of kilometers driven until the spark plug begins to misfire. Misfiring causes not only wastage of fuel, but also irregular engine vibration and output drop. In particular, for vehicles with emissions controls, misfiring can damage the catalytic converter, so using a spark plug all the way to the end of its physical service life is inexpedient economically.
Therefore, as the economical replacement interval, replacement after the number of kilometers in the figure above is recommended. (However, 100,000km for platinum spark plugs.)