The interval is for the plugs is supposed to be 100k miles.
That being said, you can hit up youtube and google for plug replacement on the 2.4 l dodge darts (same engine, same iridium plugs), of people pulling theirs and replacing at 30-40k, and most of them the plugs are in surprisingly sorry shape.
An example this one at 8 minutes gives a good look at their cylinder 3 plug. Cylinder 1 looks OK (at 2:50, also shows the champion plug's skinny electrode tip that is near gone on cylinder 3), but cylinder 3 looks end of life at 30k.
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