I think he does it after about the 6th or 7th book.
Actually MZB had some cowriters, especially in anthologies and those generally worked pretty good. Darkover is a complex enough world with a long history to write in that there are a lot of stories, and none are super long. So that’s one place where it works. Of course, those are about lots of different people in different times. Only a few of the stories were actual multipart books.
I think world books can go on for a very long time and still be original. Thieves World is a particularly good example, and there’s a lot of authors writing there.
But a single set of characters…not sure that can legitimately go longer than 6000 or so pages…other than A Song of Ice and Fire, of course. There are always exceptions.
Anyway, I love long series if there is a story being told, but if it looks like the author is just cashing in, then i cash out.