I find it depends heavily onto the task. Some let's say "organizational" work (moving stuff around, adding new machines to cluster etcetera) that just requires to be done but not "hard"? Can do for hours, will get a bit slower just coz of boredom. So the first 4 hours are still more effective than next 4 hours but not as much so, especially when I take break in the middle (bless WFH)

Programming or designing something complex to program ? Way shorter. So I usually mix and match