When I start 'em, and the reward doesn't seem to be worth the output, I stop.
And I don't usually pick 'em up again unless I have to.
Well, as I mentioned with "Das Kapital," by Karl Marx, I never did get farther than about 20 pages into it. But with "Babbit," by Sinclair Lewis, I had to read the whole thing, because I had to write a report on it in my senior year of high school. For the life of me, I honestly can't remember exactly what it was about. I seem to recall that it was about the pointlessness, tedium, and boredom of middleclass life, although I don't think Sinclair meant to reinforce the book's subject by inspiring those same feelings in the reader!
Mark