Spark Plug Life
Here is the story for mags - not sure about electronic ignitions.
It is not unusual for the plugs in a properly timed engine to last TBO so long as they are cleaned, rotated, and have the gap set every 100 hours or so. There is a specific rotation required because the sparks from the mags alternate between a negative spark and a positive spark (one plug will be negative and the next will be positive). One will erode the center electrode and the other will erode the massive electrodes. Rotation will even the wear (much like rotating your tires). Take the even plugs and put them in the odd cylinders and if it was in the top, put it in the bottom and that usually does the trick.