Whenever it's pop/entertainment literature and it bores me or the quality puts me off and similar reasons, I do myself the favor of stopping before I lose any more of my time. Life's too short, and leisure time is too short, to accept to be bored by what you pick to entertain yourself, IMO. From time to time (rarely), I give some of those books further tries later, especially series as I know of my "problems" with beginnings (couldn't get through Eye of the World/The Great Hunt or Garden of the Moon for a long time).

For other kinds of literature, those I don't read for entertainment and if it happened to entertain me as well it's a bonus, it happens more rarely. With contemporary writers/new releases more often, but with classics I generally have a good idea what I get into beforehand, and I generally stick to it to the end, though both with non-fiction and literature, it happens a lot that on the way I decide my timing for reading that book just wasn't good, and I postpone reading it for weeks/months/years.

It happened recently with the third Monaldi & Sorti that wasn't grabbing me and that I decided to keep for the summer vacations when I can spend many hours reading at a time (my summers are usually "extreme", that's when I read concurrently the "fluffiest" entertainment stuff I choose to read in the year, and the most "heavy-going" stuff).