I think even that is probably an over-estimation. I suspect most folks can work productively (as in, being intensely focused on an activity) only for 2-4 hours

I think even that is probably an over-estimation. I suspect most folks can work productively (as in, being intensely focused on an activity) only for 2-4 hours a day, and the rest of the time is just "fluff" (which may or may not be necessary for staying productive). From that perspective, regular 9-5 in an office really "steals" our time, because we can't use it on more productive things.