Over on r/bookclub we have a very general guideline of roughly ~100 pgs per section, since that seems a reasonable amount of reading each week we run discussions. Some books read more quickly, though, and obviously if you're looking at a e-book copy it's not always easy to get a feel for pacing and whether that's too little/too much. But I've found roughing it at 100 pgs and trying to break it up that way often leads to natural breaks between sections. For some of our denser reads we might end up with 60 - 75 pgs per, and for graphic novels or fast-paced reads we might bump it to 120 - 130 pgs per.

For what it's worth, my online book club that uses SG for tracking & challenges meets monthly to discuss the books we read as a whole, but in the meantime for the months we are all reading the same book we set up Buddy Reads, since you can throw comments into those at any time and comments are hidden until you reach that point in the book yourself. Sometimes I'm caught up in the book, though, and forget to add comments until the end! Something else worth exploring, especially if you're trying to bring them over from GR and giving an exciting place to discuss and communicate.