My LTR guy has long hair (down to his waist), but unfortunately with his depression and health problems he doesn't keep it as clean as he should. I liked the way he looked with medium length hair better, and so does everyone else, but he refuses to cut it (except a few years ago he did have it cut for Locks of Love. He could do that again now, since he's let it grow out again.)
My very first boyfriend also had long hair (he had it cut just before going into the Army), and one of the guys I have my eyes on now also has long hair (and is in a metal band!)
For me, it depends on the guy. I like short, medium and long hair depending on the guy. I'm not attracted to bald white guys, but I'm sometimes attracted to bald black guys. Don't know why that is, really - it just looks better to me...