By coincidence I have just read the following in Purposeful Practice for poker by Dr Patricia Cardner and Gareth James:
Your study time needs to have your full, undivided attention....set a period of time each day ( or at minimum several times a week) to work towards the process goals... For best results, you should stick to relatively short, but intense study sessions. If you can only stay fully focused for 20 minutes at a time, then start there. As you more experience with maintaining deep focus, you can build up your time commitment. But be advised that most research indicates that 4 hours of intense focused work is the maximum that most people can do.
So, basically; short, intense sessions will be more beneficial than an all day session.