If you want conscious comprehension of a book you will spend some time activating it. 20 to 90 minutes depending on what you want to know.
One can and I have gotten to a point where I PhotoRead a book and know that I don't want or need to activate it because I'm already aware of the information in that book in one form or another... Perhaps as a result of PhotoReading the book or exposure to other stuff. That I cannot say. I know I've previewed books and thought interesting this is one that I will want to activate fully. Later I PhotoRead it find it too darned easy to activate and feel I already know the book and regret buying it for a mere 20 minutes of activation. I have often wondered 'Did I know this before I PhotoRead it'?
Alex