This is a very interesting topic.
My family being from Sierra Leone, West Africa I have seen a range of African hair lengths.
My mother and grandmother both had long, afro-thick hair whereas my father's side had thin short hair.
The range of hair type and length cannot be marginalized for an entire people. That's silly.
People all over Africa have varying hair types and lengths period. It depends on care and heritage. That's like saying that African Americans all have long hair. It depends on their heritage and hair care practices! Even natural African Americans have limiting factors to their growth.
And there are different hair care standards in different parts of the continent. In general West Africans (i.e- Ghana, Sierra Leone, Liberia, Nigeria) have more European standards of length. A lot of these countries value long hair. Whereas, climate restrictions closer to the equator (Central Africa) wear hear shorter for varying reasons.
Sometimes the society dictates hair length and sometimes it is the individual.