615, 616, 617 manual says every 15K. Just did my 1979 240D after having not done in in 20,000 miles only one valve was a little tight. If you drive alot, once a year is probally the dab that'll do ya

Takes about an hour and hour and 1/2. For meputting the valve/cam cover on is always the hardest step for some reason.

Clean your feeler gauges good after use. Always replace the rubber gasket with a new one. Check your timing while your are in there.

Regards Tom