How do you know when a series should have ended? See Word-Whore Jeffe's top 10 list from Sunday.
What? That's not enough? Okay. Okay. Slave drivers.
You need only look to comic books to understand why never-ending series can be a bad idea. Borrowing from Jeffe's top 10 list format, here's my addition to the "You might be done with a series if" list:
1. You've turned every last character into a ninja. (This happened. Xmen, 1980s. No. I won't name the writer.)
2. You've killed your main character. Again.
3. OR - your mc has gone totally insane, become catatonic, or is otherwise wandering in the wilderness of serious mental illness AGAIN. But never fear dear reader, in the next installment, the mc will pull him or herself up by the boot straps and get back to kicking bad guy fanny like nothing ever happened! (Those of us who've done mental illness and needed therapy to feel normal again will hunt you down and slap you into next week if you do this.)
4. You blew up the planet.
5. You keep changing the damned rules. (No, I wasn't bummed about Disney buying out Lucas, why do you ask?)
6. You decide on a reboot and you've lost count of which reboot this one is.
7. Your main character is on a first name basis with all the demons of hell because he/she ends up there so often, but isn't a demon him/herself.
8. Nobody actually does any lasting damage - physical or psychological - to your main or any of his/her besties.
9. Your hero/heroine says 'I'm smart!', other characters say 'that mc! So bright!' and then said mc goes out and repeatedly does stupid stuff. (Yeah, you all know exactly what I'm talking about here. No. I won't name the writer. But yes. Novel turned comic.)
10. You claim that every story that came before THIS one was just a dream.
Not on the list but guaranteed to make me toss a book against a wall? Creating a character that goes against type (in the case I'm thinking of, a kid whose super power was linguistic, not beating the crap out of anybody) and who is therefore utterly unique and then killing that character off because dealing with the implications of his uniqueness are inconvenient. I'm still bitter about that one and not just because I had a crush on this fictional character. I couldn't think of a way to put that in my 'you might be done if' list.