I'd say parents might not buy an 8-year-old a book for reading it out loud at all. Regardless of length. An 8-year-old can read, and read well. He has no need for someone to read a book out loud to him. In fact, a child might well be offended by the notion of having someone read to him - he's big now, he can do that himself, and he's proud of being big and able to do things himself.
Another thing to consider: an 8-year-old might well have younger siblings. So the parents would have to divide their time between several children. Or, even if there are no younger children, there's plenty the parents need to be doing: cooking, cleaning, etc. Reading is an enjoyable and educational activity that can keep a child entertained while the parents do something else. A book that needs to be read out loud defeats that purpose.
But what about parent-child interaction, you may ask? By that age, there are many other ways parents and children can interact, ways that are more interactive than reading to a child - that is, the child too is active. Remember that a child of 8 is reading on his own, so there's no longer a need to teach him that books are great, nor read to him what he's perfectly capable of reading on his own. Instead, a child of 8, much more so than a child of 4, is capable of answering questions like "what do you think about the book you've been reading while I made dinner?" "What do you think will happen next?" etc. Those questions help develop the child's critical thinking and imagination. (There's also games etc. - I don't mean to imply that the only possible interaction is around books.)
For significantly younger audiences, children of 3-5, parents would be buying books to be read out loud. Among my friends and relatives who have children that age, I don't know a single person who goes "this book is too long, I don't want to be reading for that much time." In fact, parents are very happy to encourage their children to take an interest in reading. A small child's attention span can vary greatly, that much is true, but a child who's routinely being read to develops a longer attention span. No surprises there.
Which begs the question - why is your target audience children of ~8? What makes the book inappropriate for younger audiences?