I generally stop reading when the quality falls or it seems like it is a different writer and the story can't hold me anymore. But I always stop when the main protagonist is killed off or when the focus is shifted permanently from the main group, to another group of people (beings) in a series. I feel that I've invested a lot of time and emotion (and money) in a character or group of characters and expect to keep reading about their exploits, not some new group. A lot of authors, and their defenders, say that they're just being realistic when they kill off a character or move on with another group. 'That's life," they say. "So get over it". But I say if I want realism, why in the Nucleus am I reading a fiction book? People die every day in life. People who have been in your life for decades move away and you lose touch. If I want 'realism', I can read non-fiction or, hey, maybe I'll just go live my own sorry life.
I read for escapism, not for realism, to go places I can't go in real life, to dream of doing wondrous things and encountering people and beings I have no hope of meeting in the 'real world'. Reading fiction is all about the imagination, about taking a trip within your own mind, a trip you get to share with the author and your fellow readers and I want to hold onto those characters and those worlds until I fall asleep at night and know that they will be there the next morning to comfort me and take me on another journey the next day, and the day after that and for many days after.