Missing summary: even the most productive people can only put in 3-4 hours of intense work per day on a consistent basis.
This is not true.
Guys, the reasonable default is not "yeah this sounds similar to what EY said once," but "I don't believe you."
http://www.johndcook.com/blog/2013/02/04/four-hours-of-concentration/
And since this is the Internet, and facts are involved, our gwern turns up there also.