I'm especially interested in the statistic that great performers rarely spend more than 4.5 hours/day on their craft. I believe it, but I wonder where he pulls

I'm especially interested in the statistic that great performers rarely spend more than 4.5 hours/day on their craft. I believe it, but I wonder where he pulls that number from. Any ideas? It's the sort of thing that seems especially important to my goal of doing great science without neglecting my family and other interests (http://wayofthescholar.com http://wayofthescholar.com). The real question, though, is how do you use that 4.5 hours?