"If you don’t want to work, and don’t really care about your work, less hours for the same pay sounds great! If you personally care about what you are doing, you don’t stop at 40 hours a week because you think it is optimal for the work, but rather because you are balancing it against something else that you find equally important."

This is spot on. I think motivation is a huge factor here. Motivated workers are more capable of remaining productive over longer periods. Switching projects/tasks when you run out of gas is usually reinvigorating. Also, some people seem to have more innate motivation like an extended "honeymoon" phase when new people start a job and they are more tenacious and eager to prove themselves.

A cool project is an external motivating factor, or an existential threat ranging from potentially losing your job to being wiped out by an asteroid. As terrible as it is, fear is a great motivator. I've worked on projects so mind numbingly boring and pointless that I've had to change tasks after a couple hours because I found myself staring at my editor hating what I was doing. I've also worked on projects where I could stay motivated and productive 16 hours a day for two weeks at a time trying to meet an important deadline.

Bottom line in my opinion is there's no such thing as a broad rule of thumb for how long someone can stay productive.