Hmm, @seanblue may be able to give you more insight, as they’ve been taking part for the last three books.
For Aria, we’re tackling one chapter per week, and there are nine chapters. It seems to work well (and several people have commented that they’ve been able to catch up at chapter 3 or 4).
I believe for Kiki’s Delivery Service they took two to three weeks per chapter over eleven chapters, but that book was something of a disaster because it turned out to be too difficult - I think the consensus at the end was that if a book needs that much time, it’s simply too difficult in the first place.
I doubt it’s really useful to compare your stories to these chapters, but taking about nine weeks overall seems reasonable. You don’t want to be reading the same thing for half a year, but the fact that you have discrete stories naturally helps to break it up too.
I think you’re always going to see a huge drop in numbers after the first rush of enthusiasm. The numbers for Aria so far are 47 > 28 > 22 over the first three chapters (53 > 31 > 25 if you include those catching up), assuming all votes are up-to-date. The drop-off for Kiki was terrible, but again, that’s because it wasn’t a great pick.