What level of students are you having read this book? For my AP English class I generally assign them about 75-100 pages per week of outside reading. I would probably count on it taking them 6 or 7 weeks assuming that you are getting deep into character development, setting, theme and symbolism. The book can be read more quickly if you are not planning on getting so in depth though. I also assign "guided reading" questions, so that the students can be aware of the plot, character, symbols etc...on which I need them to focus. Such a good book!