When I first started reading, I was the type of person who would force myself to finish a book no matter if I liked it or not. However, since I started blogging, I don't force myself to read the books that I'm not enjoying anymore. I have so many books that I want to read that I just don't feel like I should waste my time on the books that I don't like. Again, this is only MY personal preference.
The main thing that makes me decide that it's time to DNF a book is when I find excuses not to read. There have been times when I'm reading a book and I will constantly try to come up with something to do that way I won't have to read that specific book. When I notice myself wanting to do other things like housework instead of reading a certain book, that's when I know it's time to add that book to the DNF pile.