There is so much variability in the kind of work people do it's hard to make a rule that fits everyone. For knowledge workers I think 40 hours is near the top of productive time you are able to wring from people. Certainly some individuals could work much more if engaged, but on the whole, without a strong incentive otherwise, I think 40 hours is a maximum limit.
I agree with you on this. It is not only task based but individual based. When I am at my desk doing tech type work I max out 'productively' at about 30 hours +/- depending on what I'm doing. But when I'm out doing farm stuff I can run 60 or 70 hours without much trouble when on the tractor etc., or a bit less when stacking hay bales.
Mentally tired, for me, is much harder to recover from than physically tired. I'm not saying I want to work 70 or 80 hours a week lifting heavy things, but this type of hourly rating system can be very subjective by the individual.
I think if there were more variability in scheduling a lot of people would exceed 40 hours on average without much issue.
If office workers had the ability to have more relaxed PTO I think it would alleviate a lot of that. But even in places where that's possible via the type of work, it's still culturally not acceptable to just take a day off here or there without a "reason."
From my personal experience it's closer to 30, sometimes less depending on the level of creativity and number of independent disciplines/specialized knowledge bases I need to draw upon and unify for a solution. After that I'll happily welcome the most mundane, repetitive and simple tasks.
