First of all, I'm surprised and flattered at those who think I can write 50-100 pages a day! Wow! Thank you. If that were true, I'd have a book done every week. Which would be awesome. But impossible. :)
I've mentioned this on before, but books don't come out based on when I finally get around to finishing. So, I can't necessarily take days off because I feel like it and go easy on myself. I'm contracted with my publisher to have books due on a certain day, and with three series, that usually means I have to turn over a new manuscript every three months. That's really fast. I have to deliver by that deadline in order to get paid and for them to meet their schedule.
So, to answer the question, my amount's based on how close I am to deadline and if I've slipped behind. Ideally, I like to write a chapter a day, which can run 2500-4000 words--or 10-16 pages of double spaced Times New Roman in MSWord. (Book pages don't correspond to MSWord pages--books usually have less words per page). I also often work on weekends.
If the words are flowing so fast that it's like I'm just transcribing from my brain, I can type ~1500 words per hour. More often than not, the book isn't pre-printed in my brain! I know what's got to happen in a chapter, but I often need to craft it as I'm going, pausing, rethinking, etc. That makes a comfortable work day 2-6 hours. 2 hours sounds like an easy day, but if it happens, it's been very intense mentally and physically (remember my injured hands and how rapidly they'd have to type).
I'm also usually revising the last book while working on a new book, which slows a new book's progress. Travel and life can also slow a new book. If I find I've seriously fallen behind, I can get a small extension from the publisher--but that only puts me behind on the next book. More often than not, if I'm running out of time, my days are getting pushed to 9 hours of work (split up throughout the day) with a 2 chapter/6-7k output.
You're probably thinking, "Richelle, most of us work 8 hours a day. What's going on?" All I can say is that the process of writing a book isn't the same as answering phones--which I used to do! In that case, I had specific instructions: phone rings, answer. Repeat. With writing, the instructions change every day--and sometimes there are problems I can't figure out, which makes it a very mentally challenging job.
Anyway, I hope that helps with the "write faster" and "write more" comments! :) And yes, to those who've asked, I'm always working at least one book ahead of you. Spirit Bound isn't out, but I've already finished Last Sacrifice's first draft--but it's a long way from being ready to show up in book form.
Thanks so much for all the great questions and comments, guys! It's fun hearing what you think, and I'm glad your anxious for more books.