Depends on what makes the book potentially bad for me.
If it's merely that it so far fails to catch my interest, but there is something about it that makes me think it may do so later--interesting subject, for example--I'm likely to read until page 50 or beyond to see if it will grab my interest at some point.
If it's failing to catch my interest and I don't see anything that makes me suspect that will change, I'm less likely to read to page 50 or beyond, but may well do so from time to time.
If it's blatant soap-boxing through the mouth of a character, or other ham-fisted approaches with similar intentions, then regardless of if I agree with the writer's stance on the issues being shoved down my throat or not, I'm almost certain to stop reading within a few pages from the point where such antics started.
If a book appears to be filled with spelling and grammar errors, I usually notice on the first page. (Excepting, of course, those books where (only) the first five pages have been cleaned up) If so, I flip through the book to a few random pages. If I don't catch any spelling/grammar errors on those, I might chalk page one up to ridiculously bad luck and read on. Otherwise, the book gets tossed aside immediately.
For other forms of (to me) annoying or bad books, it depends a lot on how much they annoy me, whether they hit specific pet peeves of mine, etc.
So basically, for me the division is between "books that leave me apathetic but with some hope" as well as "books that annoy me from time to time but that also have enjoyable parts", which get fifty or more pages and may even end up with me having read the whole book on the one hand, and "books that tempt me to throw them through the window", which tend to get a lot fewer than fifty pages on the other.