Waist length is the absolute limit IMO, I think around that point it starts looking less hygienic, whether the hair is in good condition or not. It can get to a point where the hair itself looks beautiful but it makes the wearer unattractive.

Even before it hits the waist, it can sometimes get to a point where you can see gravity making the person kinda tilt back.

On my head, I would say bra-strap length would be the limit, but as I get so much shrinkage my hair would probably be overlong if I straightened it, but the weight of the extra length would help "hold" my hair whilst curly.