A fellow I know who was into racing flushed his brakes after every racing weekend. Apparently the racing crowd knows where to buy brake fluid with some dye added. So he had two pressure bleeders each with its own color of brake fluid and he alternated which bleeder he used. He said he flushed them until the fluid came out the color of the stuff in the bleeder.
For us more everyday types, I think I'd just flush until the fluid comes out looking clean.
On the other hand, I switched to DOT5 when I rebuilt everything and, as I understand it, periodic flushing is not required with that. It has been about 12 years since I did that and I've had no problems and the fluid still looks clean in the master cylinder.