I have found similar results in my time tracking.

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.