I struggle through a book about one out of fifty times. I'm really careful in the first place which books I read because I'm one of those picky people...and I usually cycle between romance, historical fiction and action/adventure or thriller. I do get bored easily and sometimes even with books. If I have to put the book down multiple times or its taking me too long to finish I often just leave it until I become interested again and then will finish it. I also go through periods where I need to take a break. Sometimes its hard to fit reading into everything else going on in life...but I always have a book in my bag for the ride home on the bus after work.
I'll read four or five longish chapters but under half the book before I decide not to finish. If the book is terrible and I do not want to continue on I will skim through the rest of the book. I will often search out reviews of the book on Chapters, Amazon, LibaryThing and blogs to see if it was a DNF by others to justify my decision. So far I have one glaring DNF on LibraryThing and that was an EarlyReviewers ARC of Lori Handeland's Any Given Doomsday. I did do a quick review and then I traded in the book at a used bookstore. Besides this one I cannot remember my last DNF.