$13 Spark Plug I'm not goint to take it any

[Yikes]

Four years...20,000 miles?  How long can you run a spark plug?  They're certainly cheap enough to replace (if they don't come from a dealer).  Besides the mileage intevals, how can you tell when it's time to retire a plug?  Is there a way to test it to decide whether it goes into the trash can or back into the cylinder?

[Justin B.]

I pull mine out once per year, inspect, clean, and toss 'em back in if they look OK.  If the electrode is starting to get rounded or eroded, deposits on the ceramic, etc., then it's time to replace.  I have never been in the habit of replacing things just to be replacing unless it's an item with a definite life-span that will cause major $$ damage if it breaks.