I usually stop reading when I feel that my expectations are being tested too much, or just not met. For example, when a story starts out with a relatively realistic set-up and characters, but then relies on unrealistic elements to progress it. I'm thinking of all of those terrible car accidents, parental deaths, and miscellaneous hospital admissions, not to mention the lottery winning, empire building, and secret benefactor moments. If it's a fantasy (or even Fantasy), that's fine, but please be honest about that up front. Similarly, when random relatives, friends, and colleagues start 'coming out' as gay and you end up with what must the gay-est school/office/business/family/whatever ever.
Now, I'm off to make coffee, and passing as I do, a mirror or other reflective surface into which I can stare, I'll also describe myself in tedious detail.....