The book says to stay at about 2 seconds per page, although I tend to go much faster - not because I've mastered the system, but because I can't focus for that long of period of time. But I do think that 2 seconds is a good time frame.
If so - then you have 2 secs times 900 pages = 1800 secs = 30 mins (unless I'm having a bad math day). Whcih means that you're technically going even faster than what the book says.
Since I never would have the patience to photoread a book for 30 mins or even 20 mins, when I read my big 800 page textbook, I went much faster than 2 secs per page, probably more like 2 pages per sec.
I think the key is to relax, get comfortable with the state and let the pages flow past your eyes.
