To maintain a physical body in 'good shape,' I believe that it takes as little as 6-10 hours per week. A more modest estimate - or a more relaxed definition of good shape - might result in a smaller lower bound of, say, 4-8 hours. For our purposes, good shape implies moderately high levels of athleticism, physical health, and success in some sport-like endeavor. Naturally, I do not mean to be overly rigorous or exclusive here: a person who does zero targeted exercise (and, instead, lives an active regularity), can be both healthy and relatively athletic. But, I am referring to developed muscle and endurance, and honed athletic prowess. I will self-indulgently assume that I have all these characteristics in order to use myself as experimental subject:
I have come to conclusion range after observing my own weekly exercise formula. My range for targeted exercise is, on average, between 4 and 10 hours per week. The disparity between minimum and maximum hours is indicative of my mercurial schedule and way of life: I have no typical weekly exercise regimen, only regular activities: running, rock climbing, lifting weights, jumping rope, and stretching. I also have irregular activities: hiking, kayaking, skiing.
This week (Sunday March 14 - Saturday March 20) is characterized by consecutive days of rock climbing and running. I run for about one hour every other day and climb for about two hours every other day, resulting in an alternating sequence. With the aim of engaging in full-body workouts as much as possible, I always combine auxiliary kinds of exercises (e.g. stretching, push-ups, sit-ups, jump roping)with the primary activity. For instance, today I ran 4 miles in the succulent sunshine and also jumped rope, did pull-ups, and stretched. This week, my allotment of time to exercise is some 9 hours.
The argument that physical good shape exists and is something one achieve's through laborious and strenuous exercise is essentially intuitive and not controversial. My range of hours is both wide and relatively modest, and should, I think, be highly acceptable.
I follow this inductive, intuitive argument up with a question, whose answer is not as obvious:
How many hours must one dedicate to targeted cognitive exercise in order to be in good shape mentally?