Mason Curry’s book Daily Rituals: How Artists Work studies the habits of 161 brilliantly creative people, from Beethoven to Maya Angelou to Benjamin Franklin. Study skills author and Georgetown computer science professor Cal Newport reviewd 25 of those profiled and estimated that they spent an average of 5.25 hours a day in deep work—time of concentration and mental stretching, free of distractions.
Not surprising that none of those profiled became great by constantly emailing, texting, and checking Facebook all day. You can learn more about the book and Newport's findings here and here.