Not that I think it matters, but this is set in modern times, in a fictional city similar in location and population to New York City.
I've tried googling 'hair growth rates' (apparently hair grows at 0.4 mm a day, but I went to a mostly minority school and so I know black people's hair grows considerably slower than white people's hair), 'hair growth rates' + black ('Hair Growth Pills for African-Americans!'), and 'hair growth rates' + African (ditto).
I've got what I need. Thanks!