The answer is ½ inch. Milady states the average growth rate of healthy scalp hair directly: about ½ (0.5) inch per month.
The book adds important context beyond the raw number: growth rate differs across specific parts of the body, between sexes, among races, and with age, and growth occurs more rapidly between ages 15 and 30 before declining sharply afterward. The ½-inch figure is an average, not a fixed rule that applies identically to every client.
Milady Standard Barbering, 6th ed., Ch. 10 (Properties and Disorders of the Hair and Scalp), p. 255: "The average growth rate of healthy hair on the scalp is about ½ (0.5) inch per month."
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