Depends on your purpose. Textbooks on a subject that's new or working with for the semester or year, PhotoRead more than once.
Book you plan to activate PhotoReading once is enough in most cases. If you're planning to just PhotoRead 3 to 10 books a day can be the same book or different. If you notice boredom from PhotoRead the same book over and over, pay attention it means you're shutting down.
Alex