

4 hours of focused work is generally the maximum, in my opinion. I usually add 4 more hours of low-intensity work on top, to keep up with all the administrative stuff etc.
This is highly dependent on sleeping well and eating healthily, though, and on taking at least one day off per week, if not two.
I’m of the opinion that working hard is foolish, I’m focused on working well. Some days I can do 5 hours, some days I can do 5 minutes. That’s okay, no work is always better than bad work.
There is a correct way to do that but no one does it :