For me, it's just above the bottom of the buttcheeks. Aside from a woman's hair, I think the sexiest part of their body is where the butt meets the legs. To see hair cut straight across with no layers or anything just above that spot... I love it.

Don't get me wrong, if I ever were to date a girl with hair longer than that, I wouldn't even think of asking her to cut it to that length. But if some girl came up to me and said "you get to choose the length of my hair. It can be anything." That's what it'd be!