I am doing my own individual study, and i found this book somewhat helpful. It suggests 15 min per session, but the amount of work "homework" the book ask you to do is no way would be under 15 min, often it takes me 30-45 min and often i have to closed the book and put it away, and still not be done with the session. It's individual adjusment. Also not deep enough, i always have to have some kind of commentaries to answer some of the questions.