What's the longest series of books you ever read, and did the quality stay up all the way to the end?

I've read Stephen King's The Dark Tower series and ended up being disappointed by the last 3 of the 7 books.

I only made it through the first three Wheel of Time books and gave up because I wasn't getting into them.

I'm currently reading E.E. Knight's Vampire Earth series and loving them. It's up to 7 books and I'm reading part 5.

Jim Butcher's Dresden Files is another good one. I've read the first 4 or 5 of them, and it's into like the 10th or so book now.

I only read the first two in the Wheel of Time. Could see it wasn't going to end while he still had blood pumping through his veins (meaning no disrespect) and stopped reading. Now with Brandon Sanderson writing the final book I have to decide whether to just read book 12 or all the middle books I've missed. I'm a fan of Brandon's, but that's a big sacrifice.

6+ book series:

Margaret Weis and Tracy Hickman's Dragonlance Chronicles and Legends = 6 books

M. Weis and Tracy Hickman's Death Gate Cycle = 7 books

David Brin's Uplift Saga = 6 books

Orson Scott Card's Tales of Alvin Maker = 6 books and one more on the way

Robin Hobb's Farseer/Tawny Man trilogies (I don't include the Liveship Traders trilogy as part of the same series) = 6 books

Frank Herbert's Dune series = 6 books

And the longest: Lois McMaster Bujold's Vorkosigan Saga = 14 books and counting

The longest series I've read in its entirety is Oz (counting the books typically considered official books - I've read some but not all of the unofficial entries in the series). The Baum books are generally significantly better than the books written by Ruth Plumly Thompson or the other folks who contributed.

I've read nearly all of the Judy Bolton mysteries (all fantastic), but a few of the last books in the series came out after my inherited set was purchased and I haven't found them all as an adult. Ditto Trixie Belden (except they were still coming out with new books when I was a child). I've read maybe 40 of the 56 original Nancy Drew books but I've not made any real attempt to fill in the gaps. There are a lot of other long mystery or adventure series I've made a dent in, some of which were consistently good and others not so much.

If you include all of the periphery books (collections of related short stories, books in the same world but focused on other characters), the longest science fiction series I've read is David Weber's Honor Harrington series (11 books in the mainline sequence, 1 book each in two spinoffs of the mainline sequence, 4 collections of related short stories, plus a couple of stories in other collections not devoted wholly to the series).

While I have some real problems with one of the major decisions made late in the series and don't feel the more recent books are as good as some of the earlier books, I like this series a great deal. At its best it's fantastic and at its worst it's still better than a lot of the crap out there. The one book I actively disliked the first time I read it improved on subsequent reads.

Andy already mentioned my very favorite series, Lois McMaster Bujold's Vorkosigan Saga. One of my goals in life is to get as many people as possible hooked on the series. The guy who introduced me to it (and to Honor Harrington, for that matter) was a firm believer in paying it forward so I do.

I could go on, but I have to go so I'll shut up now :)

Those detective novels are individual stories. I'm talking more about books that continue one long story.