I have studied and kept detailed logs of my work for years. When I reach 4-5 hours of "real" work (i.e. deep work) I am mentally exhausted and I consider it a good day.
No one expects you to be 8 hours straight in the zone, no one does. And if you can do so, the more power to you, but be careful not to burn yourself out.
3-5 hours of deep work - that's it