Use it or lose it. Only you will be able to determine how long something stays in your memory. It it is important to your career or well being you will probably apply what you learn from a book more than remember what was in the book.
You can read a book and activate it years later just as if you only PhotoRead it the day before. Or you can feel like you've never seen the book before. Depends on the individual, the topic how important it was and still is. If it is a book for studies, PhotoRead it again befoer activating. the 3 to 5 minutes spent doing that will often make activation and learning easier anyway. If you have already PhotoRead the book you can safely add the PhotoReading step to the activation, no need to incubate again.
Alex
