Watched some red bull racing on tv, and they had g loads displayed. over 9 was very common for the duration of the turns (approx 3-6 seconds), and a few pilots were pulling over 10 for the same duration.
As for the raw physiology...
As soon as the eyes lose blood pressure, visual acuity begins to decrease. Grayout and visual blackout can occur nearly instantaneously, even if the pilot is still conscious.
The brain has enough oxygen stored up to permit approx 3 seconds of consciousness after a loss of blood pressure and/or blood flow. This is true even if the pilot can't see, because the eyes are affected first.
There is a "rebound" effect after the first G onset, at approximately 5-6 seconds after the Gs begin. If a proper anti-G strain is done for the first 5-6 seconds, the body adapts and automatically helps with the strain. If the strain is not sufficient to get to this state, then gloc will probably occur, somewhere between 3 and 5 seconds after the initial G onset.
Fatigue takes hold after 10-15 seconds, so a second risk of gloc occurs when the fatigue impact is felt after somewhere around 10-15 seconds.
I've seen (on tv) a couple of red bull racers gloc while flying the course, right around 9 to 9.5 Gs. In all cases they rolled partially wings level before passing out entirely, resulting in "bounce-and-go" impacts that left the planes damaged but flyable.
As for myself, my sustained G tolerance has hovered between 6 and 7 Gs with a G-suit, meaning I can hold 6 to 7 Gs as long as necessary without having to relax the turn. My upper tolerance for Gs has never been tested since the planes I flew were not rated over 9 Gs, and I was able to handle 9 Gs during break turns. In the centrifuge, I tolerated a sustained 7.5 Gs without loss of vision or ability to manipulate the flight controls. Without a G suit, I found it fairly easy to sustain somewhere around 5 Gs for extended periods of time while still being able to talk/instruct, and fly precise maneuvers relative to another aircraft. Without the G-suit I also had no problem sustaining approx 6.5 Gs for a minimum of 15 seconds, however above 6 Gs without a G suit I found it difficult to talk and 6.5 Gs was fairly fatiguing.
And I'm just a fighter pilot, not a red bull racer or professional aerobatic pilot. Those pilots train for even higher sustained G loads. For all of us that pull lots of Gs routinely, one constant factor is that we have all gone through fairly rigorous training to build up our tolerance for Gs.
