When I get a new book I typically will PhotoRead it on day one. On day two I will preview it and PhotoRead it again. I'll then start activating it. I seldom spend more than 20-30 minutes activating a book in a sitting.
I've learned that if the book does not gel for me in that first 20-30 minutes, it won't that sitting. I will come back to it the next day for another 20-30 minutes. Again, if it doesn't gel then, I'll wait another day.
I'm finished with most books on day two unless I'm reading for fun.