Depending on the manufacturer and/or engine type, it usually ranges from 40,000 to 100,000 miles, and from four to six years.
Many, manufacturers (such as Vauxhall) have different intervals for different engines.
Regarding the recommended service interval set on the KV6 engine, it interesting to see what George Houghton (ex Rover Group Engine Development Manager) has to say.
His team was responsible for the KV6 development under the R40 programme, that experienced premature cambelt failures.
It was identified as a combination of excessive heat under the covers and cam torsional vibration.
Naturally, you cannot eliminate vibration totally, but his brief was to engineer out this problem, to achieve an acceptable level of a target service interval of 90,000 miles or 6 years, whichever is the soonest.
I agree that there always be a safety margin built in, but anyone stating that they don't have a shelf life and last for 20 years is incorrect.
The limits are set from an engineering point of view, rather than dreamt up by a marketing department, as some posters would lead you to believe.
