It depends on the material for me, though if it's Star Trek I'll always finish it - back to this in a minute.
I've been known put non Star Trek books down, at whatever point in the story if they don't catch me, or turn me off. The most drastic example is Dan Brown's The Lost Symbol. I read through how many ever hundreds of pages and got to within about 20 of the end when the storyline so turned me off I could not limp through the last bit.
But for Star Trek books, I've developed a so-far sure way to get through a book I don't care much for. On this subject I'm pretty suborn - I want to finish. First though, I only read TOS 'centric stories so I'm most of the way home because of that. The worst time I've had with one of these is the one I'm currently reading, Vanguard: Harbinger. The biggest problem for me is there's not enough TOS crew for my tastes, but that's not the book's fault. I'm indifferent to the writer's style overall, and this book contains two of the very worst lines in any book I remember; "An acrid, musky odor clung to him like a bad reputation". I thought that was pretty terrible - more at home in a cheap '40s detective story maybe - but not as bad as this stinker, "As soon as he left the compartment, he sniffed and groaned to realize that the cloying perfume of debauchery clung to his rumpled clothes like a chigger on a bare leg". Really



!!!
Anyway, my fix; I'm finishing this book up only at work during my lunch break - I'm at work, how bad could a Star Trek book be in that setting. Otherwise, I'm reading other books in between. So far, I've read three other novels while I'm slogging through this one.