How Many in a Series is TOO MANY?

I was just clicking through Google Reader, checking out the blogs I follow, and came across this book. My first thought was, HOLY HOT HUNK! Then, of course, I wanted to read it. I checked out the deets and the blurb and found that it's the 9th in the HOUSE OF NIGHT series.



NINTH!!



And I haven't ready any of the other books, yet. That means I have to go buy EIGHT other books and read them before I even get to this one.



OY!



Now I'm thinking, not so much. Yeah, eventually I'll get to the whole series. It looks awesome and I want to read it. But it will take a while. Thing is, it's the 9th in 13. THIRTEEN!!  YIKES!



So, here's my question. 



How many is too many when it comes to a series? 



I hate to say it, but I'd have to be one heck of a devoted fan to stick with the same story for thirteen novels. Or maybe it's one of those series where each  novel has it's own story, but they share some common characters. I'm still not sure that would be appealing unless they were really good. 



Oh, and if you've read this series, let me know if you liked it and if it would be worth me coughing up the cash to get them all.