In general, how many hours of productive work do you manage in a day?
Context being this poll over on Hacker News and this article about a company with a 4-day-workweek.
Personally, I reckon that 5 is about what I manage most of the time. Depending on intensity. I can _do_ more work than that, but if I've done five hours of solid work then my brain is basically fried.
It also depends on the type of work. I can do "bitty" work, firefighting lots of little issues and dealing with other people's problems, or I can get my head down and do some complex/deep problem solving. I cannot mix the two.
(And no points at all to anyone quibbling about the times overlapping. Unless your estimate is accurate to an infinite degree, err on the side which seems more likely.)