The ideal length of a working day is max 4 hours

Engineering work is work requiring a sharp attention: one must be quick to notice new things and to handle complex wholes in thinking.

But one's attention tires easily, so if the working day gets longer, one starts to use one's memory instead more and more and becomes like a robot not capable of anything new.

So at least for me the limit between sharp capable thinking work and bored repetitious slow non-productive work goes at the maximum of four hours of thinking work a day. That is the time that our hunter-gatherer forefathers worked each day.

Anything more than four hours makes the whole working day unproductive, while a working day of a couple of hours gives full speed with good quality working results brought by a sharp attention.

So the best lenght for an engineers' working day would be a maximum of four hours.

In addition one would need to ban the same type of work during the rest of the week, because here 4+4 < 4 while 3+1 might be even better...