Spark plug replacement intervals for cars vary on the type of spark plug installed in the car. Spark plug life can vary from 10,000 miles for copper spark plugs to over 100,000 miles for iridium spark plugs.
Spark plug replacement will depend on a number of factors outside of the manufacturer's recommended replacement intervals. These factors include type of use of the car (i.e. highway miles vs. city miles), type of engine (i.e. normal vs. high performance), heat from the engine, fuel quality and oil quality.
Signs that spark plugs may need replaced are rough idling, stalling, loss of fuel economy or trouble starting. Driver should contact a mechanic to evaluate spark plug condition and replacement if any of these symptoms are present.