The Wildflowers and Marbles blog post outlines it well (the link is above).
My kids usually have 6-8 books going at once.
We mix it up, depending on the subject.
Our Geography book (Book of Marvels) we are reading 1x/week, so it will take the whole school year to get through that one.
Our literature book we are reading 2x/week, so it takes 2-3 months to get through each book.
At night, when they read their literature book with DH, they read about 5x/week, so those books only take a month to get through.
Books that are assigned for independent reading are generally read daily, so those books only last a couple of weeks.
We just finished Charlotte’s Web as part of our morning readings, and all of them are a bit disappointed to see it end, since we’ve been reading it for two months. They are anticipating what will be next, though.