Once you start a book do you finish it no matter what?

A lot of my books I purchase from a used book store. Sometimes I have a specific book in mind that I am looking for, but often times I chance it and pick something that looks interesting just to try out new authors. The book I’m reading now is well written and I love some of the prose, but I’m not interested in the story line.

The problem is that once I start something I feel obligated to finish it. I know this sounds a bit odd, but I’ve learned that sometime you have to give it a few more pages. Does anyone else have this problem? When is it time to put a book down? Is there a certain magical number of pages where you can safely assume the book has nothing to offer? I’ve read books I hated all the way to the end and then felt like i’ve lost precious hours of my life.