This question is more complicated than it seems. Without a fork, the standard length of hair at the top will vary savagely from person to person. It turns out that each part of your body has a terminal hair length, and its genetics determines that length. This is the reason why some men have hairy arms while others have barely perceptible arm hair. The same happens with his ass. On average, the length of the hair on the butt but varies from less than one inch to several inches.
When you include grooming, the standard length of the buttcrack hair is zero. Most forms of hair removal leave it completely soft.