Growing up, "long hair" was past your shoulders and before I went off to college, I knew of about 6 full black girls with hair past their shoulders, 1 of which was a little past APL. (And I come from a predominately black area btw - it's probably about 95% black.)
Being away at college, I met lots and lots of women with legitimate long hair (APL+) but they were all Ethiopian, as my school had a HUGE Ethiopian population. None of my black friends had APL hair - most had nape-length, shoulder, or a little past (I went to a predominately white school so my "friends" pretty much made up most of the population of black girls... lol... which may have had something to do with why I never saw any black girls with long hair since the population was so small <- I'm exaggerating but not by much. All the black people knew each other and I never saw anyone longer than a little past SL). Since college, I have moved to a predominately white section of DC, so I still only know of those 5-6 girls from my school days, plus one grad school friend who has about BSL hair and my cousin who is damn near hip-length at this point (both of them are natural).
ETA: My old stylist had really long and thick relaxed hair. I'm not sure how long because I honestly don't remember having a standard of measurement past SL back then since anything longer than that was just considered "long." I'm pretty sure she was at least BSL though.