At best 2 flow hours before lunch. 2 flow hours after lunch. Sometimes bonus 1.5 flow hours after ‘tea time’ (not sustainable). Sometimes bonus 1 flow hour after dinner (completely unsustainable). Basically 4hrs is sutainable. 5.5 is pushing it.
Also I’ve found if you don’t take at least a whole day off (hobbies that look like work don’t quite count as time off) then daily concentration with quickly deteriorates as well.
edit: I should note that these are high concentration hours, my data isn’t as good for non heads down work.
Productive h/w varies way too much from person to person and by circumstance. I let ppl set their own expected work schedule for the next few weeks by themselves as long as they keep the prediction up to date and their accuracy is reasonable.
I have had ppl get solid 40-60h/w effective time on target, and I have also seen <10h/w. The ppl I bring in are better at predicting their productivity and current life situation than I am. They are competent adults. Let them plan, structure, coordinate by themselves and only interfere if evidently necessary.
One size fits all is nonsense. If in doubt: measure and analyse.
So no, you're not cheating the system, you are probably running at optimum efficiency. The efficiency of the average employee in terms of productive hours per day spent in chair is ridiculously low.
As a side note: I'm very much wondering whether I should insist on 40 hour work weeks when I get to the point of hiring people. I know there is a movement going for the 4-day workweek, but I personally see more in a 6 hour work day.