I don't. When I'm clearly not enjoying a book and don't feel like finishing it, I stop and move on. Unless I have a pressing reason, I see no point in pushing myself to do something I hate. Reading is for me is fun and interesting, it is not something I do to torture myself. Sometimes it's hard to "give up" on a book, but in the end life is too short.

Do what you enjoy the way that works for you, there are no rules.