I think 5 hours of 100% engaged, mentally intense work is about all that can be expected from a person in a day. After 5-6 hours of mentally intense work, there are diminishing returns. Therefore, I think about 6 hours/day are what should be expected from a developer, including 30-60 minutes of meetings/admin work. Though some companies might need another hour or two-- hopefully such extra time is not mentally intens…
Anyways that's interesting. I actually did a personal study during the course of 1-2 weeks a couple of years ago. I kept track of how long I felt like I could give my undivided focus to my work and found that the number was 5.5 hours on average. People will be different of course, but that's as long as I felt I'd feel good claiming to have given it 100%.
Given that I had to work 8 hours in practice it felt like a bad strategy to give it 100% for 5.5 hours and then whatever (60%?) for the remaining 2.5, so instead I "diluted" that focus intensity throughout the day.
You make a good point about the diminishing returns! It sounds reasonable to me that it would likely be better to do 1 hours the next day instead of continuing by creating a mess on the remaining 2.5.
It's hard to measure, but from personal experience I'd say that it'd be worth more (given all externalities) to pay me for 100% focus over 5.5 hours compared to [whatever%] over 8.