I've PR'ed up to 10 books in a day, I find that if I do more than say 3 without a break I get a bit of a headache and need to go for a walk.
I also find that I need to break occasionally and go and write down stuff that comes to mind, it becomes distracting if I don't empty some of the ideas and thoughts out on paper. This happens often if I PR a bunch of books on the same or related subject.
