This really varies, depending on the quality of the manuscript, its appropriateness for the publishing company in question, the editor’s workload on the day she’s reading it, who it came from, etc.
Sometimes it is just obvious right from the start that a manuscript is not well-written, or is not appropriate for the editor’s list. For example, if I edit mainly supernatural romance novels for the YA age bracket, and I receive a manuscript for middle grade readers about a character dealing with the death of her mother, I don’t have to read very far to know it isn’t for me. I may think the book is very good; I may wish I had time to read the whole thing. But realistically, I have too much to read to waste my working hours on a manuscript I can’t publish. So I would move on after the pitch letter and a chapter or two.
Likewise, I could pick up a manuscript that sounds like the kind of thing I might publish, but when I get started I find that I don’t believe in or like the main character, the setup is full of cliches, the writing is clunky, or it’s just like something else I’ve read recently. Again, I don’t have to read very far to see these warning signs. I would probably check out the first couple of chapters and then move on to something else.
The more difficult manuscripts to decide what to do with are those that are really pretty good. Not great, not going to blow anyone’s mind, but you know, really ok. Often editors need to read more of a manuscript like this in order to decide what to do. Maybe the book has potential to be great, if the author just rewrites the beginning? Maybe the editor is hungry, or in a bad mood, or tired, and that’s why the opening hasn’t grabbed her. Maybe she just needs to keep going for a while and settle in to the story, and then she’ll find that it really is very good after all. Sometimes an editor will read the whole thing, get a second opinion, and then set it aside for a week or more before deciding that after all that, the story didn’t really stay with her.
Then there are the rare cases when the editor thinks she might be on to something. The concept is right, the main character is believable, the voice is excellent…that’s when the editor reads on and on with fingers crossed that nothing is going to go wrong before she reaches the end.
I personally read a manuscript until I hit an obstacle too big to overcome in the editing process. It may take me a chapter, a quarter, a half, or the whole book. But once I find that obstacle, there is no point continuing with the rest.
If a manuscript comes from an author with whom the editor has worked in the past, the editor is likely to read the whole thing no matter what, simply because she knows she will need to be able to discuss it with the author whether making an offer on it or turning it down. Often, multiple editors within a company will read the same manuscript, in order to get a consensus of opinion and a variety of thoughtful comments, even when everyone knows the book isn’t going to be acquired. But they would really only take this time on an author they had worked with before and planned to work with again in the future. If they don’t know the writer, they won’t offer this kind of feedback.
