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.
Body hair
Loss of fur occurred at least 2 million years ago, but possibly as early as 3.3 million years ago judging from the divergence of head and pubic lice, and aided persistence hunting (the ability to catch prey in very long distance chases) in the warm savannas where hominins first evolved. The two main advantages are felt to be bipedal locomotion and greater thermal load dissipation capacity due to better sweating and less hair.