Just read a very good mystery where the murderer was arrested and taken away 3 chapters in-- I mean, Chapter 24 out of 26 chapters, so 2 chapters after the solution of the mystery. I read those last 40 pages thinking that any minute, the REAL murderer would be unmasked. After all, why solve the mystery so early in the book (with 2 chapters to go), if that's the actual solution.
So of course I thought the guy fingered for the crime was actually taking the blame for his mother or girlfriend-- whoever the real murderer was-- and that the truth would be unveiled in the last chapter.
Nope. The guy arrested was really the murderer. Now I would have had no trouble with this except for the expectation set up by solving the mystery too early. We're all trained by cinema-- if the monster is killed early, then it'll come back to life or there'll be a bigger monster with five minutes left in the film.
So be careful not to set up the reader for disappointment. Resolve that external plot near the end of the book. You might want a scene or two, maybe 15 pages, to wrap up the romantic conflict or the internal conflict. But two chapters-- that's tempting the reader to imagine a new ending, one perhaps more complex than yours.
When do you end your story? Any thoughts here?