Longer the better, aside from being so long that it goes down to her ass. I think its just that long hair is a very feminine trait. Shorter hair on girls just appears for masculine I guess. Otherwise I'm not really sure why. It does depend on the girl a bit, but still I generally prefer long hair.
I'm not a guy, obviously. But long hair is generally considered 'more attractive', because it's easier to pull off and associated with femininity and youth. Look around, it's mostly older women who have really short hair.
However, some look better with short hair. My little sister has a pixie cut, and it suits her bone structure and features so much more than when she had longer hair.
Myself, I really, really love long hair. Mine is down to my natural waist already, and both the boyfriend and I agree that we'd like to see it down to my hips. I think that's be my limit, though.