For me, I just have to finish a book I've started, no matter how long it takes me I just have to finish it.
I remember once when I was a little kid I lost one of my books, I found it months later and finished it. I was reading The Host by Stephenie Meyer a while ago and I was at a boring patch so I took a break from it for a couple of months and then finished it.
Sometimes I'll take little breaks from reading the book if it's boring or if it needs returned to the library etc. but I will always finish a book. I don't think I've ever not finished a book. I'm a little obsessive about that.
What about you? What do you do?