On actual "hard thinking", like working through logic, I spent about 7 hours. There was another 7 hours writing and clarifying topics, and another 6 hours or so on classes, seminars (sort of like meetings, but productive). That gives a total of 20 hours per week on work. Beyond that, I was not productive and so I didn't do any math-related work.
If you add the administrative overhead of a job, that might be another 5 hours at most, so about 3 full workdays. I think if you are truly just doing productive, meaningful technical work, beyond 3 full days most people can't do it. People can do it for short periods of time, but more than 3 days leads to universal burnout.
Now, I'm just talking about mentally demanding, technical work. I can work longer if the work isn't very mentally demanding, but even so, not for more than 4-5 hours a day. And now I'm an independent content creator, so every additional hour I work is proportional to profit.
My conclusions after observing a ton of people in technical and nontechnical jobs is that beyond working 20-25 hours a week, having people do more is useless, especially when it comes to fostering people in the long-term (short term is different).
I feel the only reason why we have 9-5, Monday to Friday jobs is because of infantile narcissists who have no other purpose in life than to go to work and push people like machines.