Spark plugs can last far longer than the stated service intervals suggest. In fact, if the engine’s running correctly they can last almost indefinitely. Check the gaps every 3000 to 6000 miles and clean them with a brass-bristled wire brush, if necessary. If they’re showing no signs of damage or erosion, they’re good to go again. If the engine starts to run on three cylinders and you trace it to a faulty plug, change the whole set.