Incubation is not totally critical to the process, and Preview/Rapid Read are two steps that are actually completely optional. Once you get it down to Prepare/PhotoRead/Activate, 30 minutes actually makes a lot of sense.
All you're going for is to get the core information out of the book. Prepare can take seconds, PhotoReading step 5 mins or less, and then that leaves 20-25 minutes for a nice single activation pass to get the core information from the book.
Bah bam, son! Your work is done. =]
It probably consists of SuperReading - going through to get the key information from each chapter. I tend to have a tougher time getting good comprehension (or enjoyment) from a novel doing a single 20 minute activation pass, but for any nonfiction book that can be all I need. That is not to say that I haven't successfully activated novels in 30 minutes or less - it's just a rarity for me because of a number of reasons.
Incubation is recommended to be AT LEAST 20 minutes and ideally a nice 24 hour period before coming back to the material. For your school textbooks - PhotoRead those suckers once a night, followed by PhotoReading whatever chapters you're studying at the time in addition to the entire textbook.
Hope this helps.
