I have, on occasion, accused some of greatly exaggerating or at least innocently over estimating how long any given soap lasts and have had several very experienced members confirm that longer soap life is achievable. However, I would imagine that 40 would be the extreme outer limit for VDH in the hands of the most talented lathermeister.
I tend to agree, OldSaw. However, I would believe that someone could get 18-20 shaves if he knew how to make just enough lather for a shave without quite as much excess. I would expect the outer limit to be somewhere around 25 shaves by the average DE shaver.