I'm not aware of any specific change interval published by Mercedes.

You should probably inspect and clean the cap/rotor and check plug wire resistance at 60K and then every 15K interval after that or at least every 30K, which is the spark plug change interval.

Engine misfires or busting an emission test with very high HC (well over the limit) with acceptable CO indicates misfires, which can be caused by deteriorated secondary ignition system components.

Duke