For both guys and girls, it depends on what suits their look best. I know guys that look pretty awesome with longer hair, and other guys that just look horrid with longer hair (my ex being one of them).
On myself, I like my hair either the usual cut I get (which is just a bit passed my shoulders), or when it's growing out and gets to my boobs. When it gets passed my boobs, I don't like it so much.
I keep it in layers so it looks nice always <3