Centrifuge and Flying studies have identified man’s tolerance to +Gz with reasonable accuracy. Figure 1 is a +Gz v. Time graph demonstrating the tolerance to +Gz of relaxed subjects not using any G-protection device or manoeuvre.

The area above and to the right of the solid black curve represents the +Gz and Time at which unconsciousness (on average) occurs. The area between this curve and the grey curve is the region of visual disturbances (greyout and blackout) without loss of consciousness. Below and to the left of the grey curve is the +Gz/Time zone where no visual symptoms or G-LOC occur in the average, unprotected person.

The line ‘C’ on figure 1 represents a gradual onset of +Gz at a rate of around 0.5G per second and shows that visual symptoms are likely after about 5 seconds and Loss of Consciousness about 1 second later at +4Gz. Line D shows a slower rate of +Gz onset, in this case visual symptoms will occur after 16 seconds (+4Gz) and G-LOC will intervene after 22 seconds when the acceleration will be +5Gz. Rapid onset of sustained +Gz, as shown in line B will result in G-LOC after about 4 seconds without any warning visual symptoms. However, very rapid onset +Gz that is not sustained at a high level, line A, may well result in no visual disturbances or G-LOC. This last feature is what saves many of our unlimited aerobatic pilots from suffering G-LOC more often, although they pull substantial G they do so for only very short periods

Full article HERE. 

Of note, you can also get ‘redout’ with negative G’s…

h/t Uncle Jay