A while back, Nat Torkington observed that there's only so many more books one will be able to read before their demise, so one must choose wisely. And that number might be fewer than you think!
But there's a tendency to want to finish a book, even if it's not what you hoped it might be. Kelly Jane Torrance, writing Closing the book on a bad read - Washington Times also urges you to bail out of a bad literary journey. Scarcity (your remaining time) and the law of diminishing returns are both factors. Unclutter's take on it is that you'll have fewer books around the home, too.
Unfortunately, I don't practice what they preach as well as I should. Personally, the last book I gave up on was The Shape of Things to Come, which is chronicled here. Before that one, it was Prisoner of X, which I found to be repetitive and poorly organized.
So, I've been inspired to do a little book purging. Those that I can't find local homes for will end up on Bookmooch.