Human hair growth
The three stages of hair growth are the anagen, catagen, and telogen phases. Each strand of hair on the human body is at its own stage of development. Once the cycle is complete, it restarts and a new strand of hair begins to form. The rate or speed of hair growth is about 1.25 centimetres or 0.5 inches per month, or about 15 centimetres or 6 inches per year.
Cass Ole
His mane as seen in the two movies was partially enhanced. Like many American horses, Cass Ole had his mane trimmed into a bridle path. While he did have a long mane typical for his breed, its natural length was about equal to the width of his neck, which is the average maximum length that a horse's mane will grow when not specially groomed. Therefore, to hide his bridle path trimming and to create the long flowing mane that was seen in the movie itself, hair extensions were stitched into his mane to provide a fuller and longer look on camera. To grow a mane past its natural length requires it to be kept in braids, regularly conditioned, takes at least a year's time to achieve significant length, and, like humans, some horses' manes simply will not grow beyond a certain length.