Most Helpful Guy
I once knew a girl who had long golden hair that went down to her knees. It was so beautiful, but she said using the restroom was a serious pain. So I suppose I would say "until you have problems taking a dump" is a good point to get it cut.
Personally, I'm not too particular. I'm more concerned about how the hair is done.
More particularly, there's this style girls seem to be doing a lot these days where they roast themselves in a tanning bed, bleach their mid-to-long hair bright blonde, then pull their bangs back so it makes this huge "poof" on top of their heads. (A little like this: https://goo.gl/ypZgcM) Then they proceed to 'compliment' their now-glowing eyes with a nice duckface. Kills me every time, and not in a good way.
But you know what looks fantastic? Short hair on a thin girl. WOW. But it's pretty easy to screw this one up as well: You take advice from Miley Cyrus.
But here's a GOOD example:
That was just from a google search, and this girl stood out. Sorry to whomever it is. (Also, you are freaking adorable whoever you are.)
Short hair has to be my favorite when done right. But curvier girls benefit more from longer hair, especially with waves. A girl I knew was a little bigger, and she had wavy, fairly long hair and just the best smile ever. I certainly found her attractive. But I mean, I was also friends with her boyfriend, and they were too awesome a couple.
Most Helpful Girl
I like my hair between waist length and right above my butt. Anything less and I feel naked LOL. I've always kept my hair long (except for a time or two with a scissor happy hair stylist.) I look best with long hair so waist length - butt length hair doesn't feel too long for me and it suits my look. Anything shorter than waist length feels short to me personally.
I don't really care how other girls keep their hair though. Just because I like long hair for me doesn't mean that I think all girls or that even most girls should keep their hair super long. Some girls look prettier with shorter lengths. But probably hair that falls at least right below the shoulder blades (bra strap length) is what I think looks best on most girls.