How Often Do You Give Up on a Book?

I just stopped reading a book after only reading about 25% of it. It was dull, half baked and overall it was pretty pointless.

Part of me feels like I'm wasting money by abandoning a book that I paid for before I'm even halfway through it. Part of me feels like I'm wasting time reading a book that bores me.

Do you finish every book you start? If you don't what makes you turn away from a book mid read? I'd like to know because I want to avoid doing things in my own writing that turn people off.

Thanks in advance.
G