When starting a new book, video game, or movie, there's a certain investment of time before which I'm fine with giving up and cutting my losses, but after which I feel obligated to continue until completion.  If a book doesn't catch my attention in the first 5-10 pages, it probably won't no matter how much I read.  Video games and movies are usually easy to judge in the first 15-20 minutes, though I'll often let them go on a little longer just to make sure.  But sometimes I'll get past that point, continue reading/watching, and realize much later how poor of an investment of time I just made.  And that sucks, because I know the time I invested in the Spiderwick Chronicles can never be gained back.  It's gone forever.  Just like my self-respect, after having admitted to the world that I watched the Spiderwick Chronicles.