On your quiz, the average answer is "3-5 hours," and this squares with recent research suggesting that in an 8-hour day, mind workers are doing actual work about 3 1/2 hours on average. The rest of the day is spent on snacking, gossiping, Facebook, etc.

I'm a highly motivated independent developer and writer, and I've been studying my own work habits to try to become more productive. I use toggl — https://www.toggl.com/ — to track my time throughout the day. I'd work 12 hours a day if I could, but I can't. I run out of gas after 2 hours, 3 hours — and sometimes 8 hours. My limit on tedious, fussy, boring tasks is about 2 hours, after which I'm not much good for anything. My limit when I'm writing a book is 3 to 4 hours. If I'm building something out of code or working in Photoshop, sometimes I get addicted, and then I can go 8 hours. Addiction seems to be the only thing that gets me close to being as productive as I'd like to be. Sadly, I can't choose to focus only on the types of tasks that I'm addicted to.

My sense is that most people — probably including your boss — have never measured how much cognitive work they themselves can do before their brain is fried. I'm talking about real work, not meetings. Meetings use almost no brain glucose at all, so are a way to pad "working" hours without increasing fatigue substantially. I'm convinced this is why meetings are so popular in corporate America.

As long as no one is measuring, it's easy for everyone to kid themselves that they're good for 8 hours and that everyone else should be, no matter how unrealistic that may be.