When the are burned out.



Yes, that sounds like a smart allec answer. But it's really hard to say. I've had plugs burn out in 10 seconds, and then I've had plugs that have lasted 4+ years (I say plus because it's still running). There are a lot of factors that are involved in it. If you run your engine rich the plugs will usually last a long time, but a lean run can lead to a short plug life. Also, new engines tend to go through plugs quickly. This is because the metal from the engine breaking in will burn out a plug quickly.

Hope this helps some.

Ken