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.
Long hair
1972, it was estimated that 24% of American women wore shoulder length hair or longer (44% of women in ages 14 to 44), meaning slightly more than 12% of men and women altogether. Similar frequency was found in 2001, when it was estimated that about 13% of the US adult population, male as well as females, has hair shoulder-length or longer, about 2.4% have hair reaching to the bottom of the shoulder blades or longer, about 0.3% have hair waist length or longer, and only about 0.017% have hair buttocks-length or longer. By extrapolating the above data and the number of hair length records, the number of people with shoulder-length hair or longer in the USA is estimated to be 27 million, those with waist-length hair roughly 900,000, those with buttocks-length hair to be 40,000, with knee length hair to 2,000 and with ankle length hair to 70.