Anya, it all depends on the quality of her hair. I'm sure you've seen some very long hair but the last foot or so is thin and straggly. If it's quality silk, thin or thick, I think anything from knee to ankle is the best. And I agree with you when you say bra strap length is not long. I'd call that medium. Anything past waistline is getting long. That said, I've seen great hair of many lengths. When the hair reaches its genetic 'end' you'll know. Go from a couple inches shorter than that and I think the woman would reach the maximum length for her hair and still look good.

Of course it goes without saying, when a sexy woman like you has long soft hair she should use it well like you do!