It varies by age -- very rarely see anyone over 30 with long healthy hair.
I do see younger (teens and 20s) girls at MBL who have presumably been covering with weaves and take their hair out every once and a while. Their hair looks fairly healthy but sometimes dry and in need of a trim to BSL - nothing awe inspiring. (I wish that young AA girls knew just how easy it is to grow their hair at that stage in life. If they did, they would double down on the hair care and leave the weaves and direct heat alone.)
One of the few times that I saw a woman over 30 with WL hair, I stopped her and got stylist info. She said that she was natural and flat ironed (hard w/ a lot of heat) every two weeks faithfully and that doing so over several years had greatly improved the health and length of her hair. I went to her stylist once and loved the sleekness and swing but concluded (after smelling my hair) that it was way too much heat for me.
I had an aunt (not on "mixed" side of fam) who had WHIP hair well into her 60s. She was relaxed and roller set faithfully (taught me how), never used ANY direct heat at all and was extremely conscientious (okay, obsessive) about hair care. She once made her husband drive her all over town in a blizzard looking for a particular conditioner.
I saw a 30+ woman at Whole Foods one time who had natural WHIP hair in two air-dried looking braids. It looked thick and she was a darker skinned woman but something told me she was mixed. She was also a raw-foodist. (Come on, you KNOW I asked. LOL)
I remember running into an ex boyfriend's older sister (she might have been in her 50s and I hadn't seen her since she was around 40) and her hair was WL. Whereas her 3(b)-(c) ish looking hair had always been a wild, straggly mess that never went beyond CBL, it was now sleek and healthy and beautiful all the way to the ends. I saw her in the salon and noticed that it was curly wavy when wet (so no apparent relaxer - maybe tex-lax?) and after she had it set on huge black rollers, it was completely straight. Maybe she just got hip to roller setting. I also noticed that she seemed to have become a little compulsive/picky -- telling the stylist not to use a certain product, etc.
Bottom line -- it is rare. Even when you see groups of Ethiopian women who typically have gorgeous thick shinny hair, it is rarely MBL or WL. The only people I see regularly with hair past MBL who have hair that remotely resembles AA are Dominicans and I know for a fact that a lot of the things that the stylists do to AA hair (e.g., extreme high heat blow out on relaxed hair!), they don't do to their own. On the other hand, to be fair, Dominican products, detangling techniques and re-introduction of roller sets are probably responsible for a lot of increased length by AA women.
One thing that WL women seem to have in common (other than a lot of roller setting), IMO, is that they are quietly passionate about their hair -- they don't go around preaching and reprimanding, but if you watch and observe the way that they care for their own hair -- it is obvious that it commands a lot of their time, energy and focus.