How many hours are enough?
As an assistant professor, during the school year no number of hours is enough (apparently). Besides math, there are the hours and hours of student contact, class prep, committee work, preparation for committee work, learning about pedagogy, grading projects if not grading homework, grant-writing, etc. Sixty-hour weeks easy. A lot of these things have deadlines (some harder than others) and these deadlines pull us into action.
In the summertime there is plenty to do but it is all paced so differently. I can spend all day on math alone if I like. Some people say four hours of good mathematical thinking is enough. I can believe that. I don't think I'm currently putting it in. Maybe if I did that would be enough.
Today I've chatted with collaborators and looked over some relevant papers. I have not proved anything. I should put in another 3 hours to reach my somewhat-arbitrary four hour declaration.
I think I need caffeine for that.