Series That Go On too Long

Have you ever read a series that you loved and you never wanted it to end? You read each one in such quick succession that they all seemed like one big novel. Then around book 7 or 10 or maybe even 16 you start to read out of some sense of duty. You have invested so much into the characters and you want to see how things end, but it starts to become a chore.
In paranormal fantasy, series are the norm and I prefer a series as opposed to a stand alone book. Too much of a good thing does exist. Sometimes characters become so powerful that there is no suspense. You know they can't be hurt. The main character sometimes becomes a parody of their former self.
I will give an example. Laurell K Hamilton, The Anita Blake Series. It turned from a book with imagination, suspense, vulnerable characters to in my opinion, porn. Don't get me wrong, I like porn. I read porn, but this series was really good. It is disappointing that it went on so long.
I went through the reading a few out of a sense of duty and have missed the last few. I just couldn't handle it anymore. As a writer I know Hamilton has every right to do with her character what she will, but Anita became a parody. Lets face it she fucks what maybe 30 or 40 men and she still pretends to feel shame. Come on please. She is better at everything than everyone else. She is pretty much every creature you can think of all rolled up into one. Good characters are good because they are not perfect. They commit wrongs, they have chinks in their armor, they are weak sometimes.
It is time to end the series. I hope I know when to end mine. I hope my readers will still want more when I bring Cassandras saga to completion. I don't want to jump the shark. So please readers if you see a fin in the water let me know. I will at least promise to think about it.