Most people only replace their spark plugs only when their engine start rough idling, misfiring, etc. But what they don't realise is, by the time a spark plug gets that bad that it starts misfiring or fails completely, it would have introduced other problems as well, such as reducing lifespan of Catalytic converter, building up carbons on combustion chambers,
The good thing about Peugeot cars with ES9J4S and ES9A V6 engines is that they have a way of telling the owners when Spark plugs are due for replacement, without total failure, misfiring, rough idling, etc.