The effect typically lasts for hours, even overnight. The duration of the effect can be changed by consumption of coffee (and psychoactive drugs). One report claims it is stronger in extroverts than introverts.
To experience the effect, spend the next five minutes gazing at the image of two colored grids on the next page. You need not focus on any one spot but it is suggested that you vary your focus from the left to the right grid and back again over this test period.
Then, look at the next pair of grids. These are black and white grids, not colored grids. But they will be perceived as colored grids as a result of the McCollough Effect. The intensity of the effect will vary between individuals. You can make the effect more intense by returning to the adaptation grids for a period of time.
The effect is not a simple after image effect. If it were such an effect, then it would not be dependent upon the orientation of the grid lines, nor their spatial frequency. In addition, a simple after effect due to the fatigue of specific color receptors should fade. But if you return to these grids the next day, you might very well still see the effect.