Double row units like in the 911 air cooled motor or the Porsche M96 engine will easily go 200k. The single row units just are not that robust, even doing duty between cams(look at MB's experience).On BMW engines, I think its worth replacing them between 100-120k. Depends on the make and the stress their under by design.
You might give Carson an email and pop one in. I think you'll find the torque curve and the character of the car completely changes(for the good in every respect).
Regards