When do the spark plugs need changed

Spark plugs like any electrical device become worn over time. Spark plugs also work under extreme condition. The use of cheap fuels will leave deposits, poor maintenance, bad air filters, old plug wires, all these items can cause a spark plug to fail or not perform at its peak level. Also spark plugs use a gap or Electrical bridge that is used for high voltage to arc across. The larger the gap the hotter the spark until you reach a point that the coil can not bridge the gap and then you get a misfire. Also when a plug gets deposits from either oil, poor fuel, carbon this will shorten the gap and cause a weaker spark resulting in poor performance, wasted fuel and engine dieseling or Run on after the ignition is turned off.
These are the simple reasons for spark plug replacement.