Interesting point. I guess, as an extension of this, I have noticed I'm accomplishing the same amount of net work in fewer hours. I can safely say I love my work, and that's definitely a driver in -wanting- to work more, but because it's exciting and fulfilling. (Obviously still balancing my non-work/family life with this.)
Personally, my only goal in doing all of this is to create great things which are fulfilling for me and others to use, and that hopefully have some lasting/tangible benefit in my/their life. In a sense, I've already achieved that. The idea of great wealth is not a driver (and I think would only distract me.)
I'm not sure I buy your ability to understand your own productivity (people are notoriously bad at that). It's hard to believe that more work doesn't strongly (but certainly imperfectly!) correlate with more output... Up until that work exhausts you to the point and you start doing BAD work. Maybe that # is 35h/week. I suspect it's widely varied based on type of work, type of person, depth of love for the work, diet, exercise, etc.
I look at it from a standpoint of what tangible things I need to accomplish, and what I estimate I'll need in terms of time to do them. In the last 3-6 months especially, I've been constantly -under- my estimates. This correlates with starting to work a shorter week, taking more frequent breaks, etc.
Not scientific by any stretch, rather simply my own observations.
And yes - I'm thinking more in terms of folks working insane amounts of hours, at levels that put them well into the "doing bad work & burnt out" realm.
Personally, my only goal in doing all of this is to create great things which are fulfilling for me and others to use, and that hopefully have some lasting/tangible benefit in my/their life. In a sense, I've already achieved that. The idea of great wealth is not a driver (and I think would only distract me.)