The book clubs run on a weekly schedule (I think, usually?), so they try to read a certain number of pages per week. That said, you don’t have to finish them by a certain date. I just finished my first book club, the ABBC’s Card Captor Sakura Volume 1 club, and there were a couple weeks where I fell behind a bit. The earlier weekly threads were still active even during later weeks, so if you do fall behind, you can still post questions/discussion points on the older boards and get answers there.
There’s a brand new ABBC club that just started, in case you wanted to check it out. This is the home thread for it, which will have links at the beginning to each of the weekly threads: ドラえもん ・ Doraemon 🤖 (Absolute Beginner Book Club) [reading starts Dec 14th!]