When the pieces are unloaded and are outside the kiln the temperature shock will make the glaze crackle after a while. There is no standard duration time for the temperature shock. If you listen carefully you can probably hear that it happens. This can be after just a couple of second for smaller pieces, and longer as your work gets heavier. It shall be obvious that the small toothpaste tube will cool off quicker that this grizzly bear sculpture and therefor needs a short time out side the kiln and before the reduction. In general you should start the reduction when the glow disappears.