Or, how much is “enough”?
Personal statement of belief, one each, coming up here.
I believe that if you’re going to criticize a work — by which I mean, do a serious and in-depth analysis of its merits and its flaws, or a serious and in-depth examination of some aspect of it, then you have a moral responsibility to read the whole thing. And that responsibility includes continuing past the point where you are certain that you don’t like it and are never going to like it and would prefer that nobody else ever like it either.
Serious criticism is serious business, and sometimes that means sucking it up until the bitter end.
I believe that if you’re going to review a work — by which I mean, provide other readers with a read/don’t read recommendation — then you really ought to read the whole thing. And if you simply can’t bring yourself to go that far, you have a moral responsibility to let your readers know how far you made it before you had to stop. (“The [insert bad stuff here] hit me in the face on the very first page, and as Dorothy Parker recommended, I threw the book aside with great force” is a legitimate review. So is something on the lines of, “I stuck with it until the last third of the book, and then the cumulative [insert bad stuff here] overwhelmed me and not even interesting characters and a kick-ass plot were enough to keep me going.”
If you’re going to vote on something, I think that you should at least look at everything on the ballot. This isn’t as onerous a task as it appears, because frankly, for most stuff it doesn’t take reading the whole thing to determine whether you think it’s make-the-cut-worthy or not. Most short fiction shows its true colors inside the first few paragraphs; most novels, inside the first fifty pages if not sooner; and I believe that in this case you don’t have a responsibility to continue past the point where the work trips your personal “life is too short to keep on reading this stuff” trigger.
Also: If you’re basing your public “will read/will not read” comments off of somebody else’s reviews, reactions, or analysis, say so, and link if possible. Clear citation is a positive good.
Dr. Doyle's Blog 
I like what Mortimer J. Adler and Charles Van Doren said about “Criticizing a Book Fairly” and “Agreeing or Disagreeing with an Author,” in How to Read a Book.
For my part, I sometimes think that one of the most useful things I learned in the course of getting first a B.A. and then a Ph.D. in English was how to recognize a well-written example of something I didn’t particularly like. Far too many readers out there confuse “I didn’t like this book” and “This is a bad book.” Sometimes they’re the same thing . . . and sometimes they’re really, really not.