I think 15-20 hours of work a week is natural. That is the amount the Kung Bushmen have been known to work per week gathering food. The rest of the time is leisure.
When at work, I use a timer to time the time spent in actual productive programming. I find that if I start to go over 20 hours of productive work a week, I start to become miserable and my body reacts by bringing on colds or allergies.
Perhaps your friend chooses to work at a grocery store because he can't find a job in his field where he would have the opportunity to only work 15 hours.
A 25 real-hours-of-work week ends up being a pretty productive and intense one for me. I feel lazy with such a low number, but I bet most office workers do far less.
When at work, I use a timer to time the time spent in actual productive programming. I find that if I start to go over 20 hours of productive work a week, I start to become miserable and my body reacts by bringing on colds or allergies.
Perhaps your friend chooses to work at a grocery store because he can't find a job in his field where he would have the opportunity to only work 15 hours.