The Fever Series discussion

How many books can be in a series before you get a bit bored of the topic? I'm currently reading Shadowfever by Karen Marie Moning and there are 9 books in that series. I'm on #5 and I'm getting winded. Not to say it's not a good read, it's just a long series. I read, I guess am reading, A Shade of A Vampire series by Bella Forrest, since I'm on #8 of 50-some books, which I do not plan on reading. Good series, too lengthy though. I like a trilogy or even 4 book series(a quadlogy?)