The end of a series. It's a weird animal. It's slippery. Not easily defined at the beginning of the series (unless you are a super freak plotter with OCD and a completest messiah complex) it is something that you either see coming a mile away or it sneaks up and ambushes you while you're looking the other way.
The best rule of thumb is to quit while you are ahead. Get out while the gettin is good. We've all seen shows that have lived past their expiration date and the last seasons (Scrubs I am looking at you here) totally suck. The characters have done all they were meant to do and so they start acting like someone else entirely. They do dumb shit and behave in stupid ways that just make you hate them.
You never want the reader to hate your character if they are someone they used to love.
I'm 3 novels and 3 novellas into my Deacon Chalk series. I am about to start book 4 soon. I know exactly how Deacon will behave in book 4. It will be natural for me to write it because he is in the midst of his character arc. I know where his arc is going and what I am going to do to get him there.
Now if I write past that arc I will probably be having to force the story and then the books will be like the last season of Scrubs and it will suck major balls.
I'll set the books on fire first I promise you that.