So long as you have food in your mouth, you have solved all questions for the time being.
You cannot raise the standard against oppression, or leap into the breach to relieve injustice, and still keep an open mind to every disconcerting fact, or an open ear to the cold voice of doubt.
At the very end of a book I can manage to work for longer stretches, but mostly, making stuff up for three hours, that's enough. I can't do any more. At the end of the day I might tinker with my morning's work and maybe write some again. But I think three hours is fine.