You know the drill - If it's worth saying, but not worth its own post (even in Discussion), then it goes here.
Notes for future OT posters:
1. Please add the 'open_thread' tag.
2. Check if there is an active Open Thread before posting a new one.
3. Open Threads should start on Monday, and end on Sunday.
4. Open Threads should be posted in Discussion, and not Main.
Do you have a link?
See pg. 391-392 of The Role of Deliberate Practice in the Acquisition of Expert Performance.pdf), the paper that kicked off the field. A better summary is that 2-4 hours is the maximum sustainable amount of deliberate practice in a day.