Depends on what your personal definition of work is.
When I'm coding, focusing on just one deep problem, and the world around me is quiet, and I have food ready, and no one is bothering me, I might be able to do 6 hours at most. Afterwards, it starts to smell like fried brain.
When I'm coding light tasks, 3-4 hours, maybe 5 on a good day when I'm rested and content.
When I'm doing managerial tasks..... I've done 16 hours of continuous work, but it's "social" work, meaning no heavy logic problems, but rather relying on my personality and intuition to solve issues or juggle tasks for others.