Regular haircut
Human scalp hair grows on average about one eighth inch per week or one half inch per month. Most clipper guards are numbered in eighths of an inch. The number of the guard denotes the number of weeks of hair growth left on the scalp when a clipper with a certain numbered guard is guided over the head with the guard in contact with the scalp. A #1 guard leaves 1/8 inch (3 mm), one week's growth of hair, on the scalp; a #2 guard leaves 2/8" (6.3 mm), two weeks' hair growth, on the scalp; a #3 guard leaves 3/8" (9 mm), three weeks' hair growth on the scalp; and so on.
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.