Most Helpful Guy
When hair gets to about half-way down your back, that's probably as far as you want to go. Anything longer tends to look strange and abnormal.
I've seen some girls with hair down to their butt, and even down to their knees. It was actually really disturbing and unattractive.
Ideal length is shoulder to mid-back. Really short hair (shaved) only looks good for the few people who can pull it off. Often it looks too boyish.