I think it was probably when she had sex with Byron in ID. The whole book up to that point had consisted of sex scenes and setting-up-scenes-for-sex-scenes, but that in particular just felt so pointless. Anita's sex life had begun to resemble George Mallory's oft-quoted reason for attempting to scale Everest: "Because it was there."

Since it's not in my nature to give up on books, and because (even taking into account other quality slippage in the preceding several books, which up to then had seemed more subtle to me) I still thought there must be something worthwhile going on somewhere, I finished it. And then I discovered that there really was no structure-saving thing happening at the end. So... curious though I was about what Anita would do next (I'd imagine some sort of liniment for the vaginal inflammation would be a good start), I felt that wasting another tenner on the next book would be just plain daft.