I know for sure it was in 1991. The Quadra 900 was my first business mac, received late november 1991 (in Europe).
As far as I know the Quadra was the first with System 7 which gave (not too much) trouble as a graphic machine (Quark XPress, imagesetter, scanner).
Although it was the most expensive option it looked to us as the fastest pc on earth, at that time of course. More like a minicomputer and a hell of a lot faster than the (milliondollar) pixelboxes.
October 18, 1991: Quadra 900 Introduced