Earlier quoted context omitted.
Thats interesting because 4 hours is exactly the limit I've seen cited by researchers as the max for highly focused work but even that much is a kind of skill that has to be developed, so a beginner might have much less concentration.
Can you give a reference for that number? Sounds intriguing...
http://nautil.us/issue/46/balance/darwin-was-a-slacker-and-y...