Never.
For me, anyway. But I'm kind of biased- I adore pretty much his entire catalog.
I'll admit that The 80's albums (excepting Scary Monsters) are weak, except for a handful of songs. But how can you say that Bowie ended at Ziggy? What about Station to Station, Lodger, Scary Monsters?
I'll go even further and say that Bowie's post-90's albums have nothing to apologize for (with the possible exception of Black Tie White Noise). I adore Earthling, Hours, and most of Outside. Heathen was a perfectly measured statement on who Bowie was at the time of recording.
Reality scared me, though. If only because it followed Heathen; it sounded too similar, and much too safe. If his next album follows suit, I'll start considering that Bowie's "losing it".
This all just my opinion though, of course.