I agree about proportion. If the hair is dominating you and making you look like Cousin Itt, it's time to trim some off!
For me personally, best length is just below the boobs. I don't want to cover up my waist. It does balance out my hips though!
never had extremely long hair but I think it would be a real hassle a lot of the time. I do know a couple of people with really horrible too-long hair (one to the knees, and it's thin and straight...bleah.) Just above the waist would be my longest limit.
I also had a friend with very long hair that she said strangled her in her sleep; another that always got it caught in everything and once someone cut a piece out of it at the movies (i t was over the back of her seat); didn't see her for a while and when I did, she had really short hair like a Louise Brooks bob.