I decide at 10% if I'm going to read the rest of it. It's rare for me to bin a book off at 10% but it has happened (usually Kindle store 99p shite). If I decide to finish it I usually try and struggle through to the end, unless I really really hate it. Its usually old stuff that trips me up in that respect, I always have good intentions of reading the classics but I just can't get tuned in to old fashioned writing. Wuthering Heights is about the only exception, but I have a long list of unfinished classics. I'm an impatient reader, I like to race through and find out what happens so slow-paced stuff with loads and loads of dialogue or description sends me to sleep.