There is a length that I term the "long hair dilemma" where the girl can't wear it down without major problems of getting caught in things, etc. Generally around tailbone length. So if she wants people other than her inner circle to see it displayed on a consistent basis, that can be a real issue.

It becomes her choice, if she wants to grow it forever and just have it down for making love, that's a fabulous fetish. I worked with a woman for 20+ years she had a huge bun every day and never wore it down, and always said "only my husband gets to see it". Sikhs and Pentecostals fall into this as a category.

From a more practical standpoint, you can get an awesome hairjob from waist length hair.

I think for us fetish guys, knee length is sort of where it goes from "long" hair to "OMG long" hair.