I've seen knee length a few times. Once on an young Indian woman at an amusement park. Absolutely beautiful and so thick - I didn't know hair could be that thick.

Once in Las Vegas there was a woman staying there with hair to her knees, in decent, but not great, condition. Saw her a few times in the promenade.

Lastly, I saw a woman in my local department store about two years ago just standing in an aisle with incredible hair past her knees. She was a bit older and heavier-set, but her hair looked like it had just come from a salon. It was down, full and rich, and covered her like a beautiful cloak. It was super thick and mostly brown with just a hint of silver streaking through. The ends had been curled a bit. She was just standing there, smiling knowingly. I might have made a few excuses to pass by that aisle again and again...