On average, fewer than half of the books tested were finished by a majority of readers. Most readers typically give up on a book in the early chapters. Women tend to quit after 50 to 100 pages, men after 30 to 50. Only 5 percent of the books Jellybooks tested were completed by more than 75 percent of readers. Sixty percent of books fell into a range where 25 percent to 50 percent of test readers finished them. Business books have surprisingly low completion rates.
That is from Alexandra Alter and Karl Russell at the NYT. There are good visuals at the link, and note this is data from ebooks, not physical copies.