The normal cycle of hair growth lasts for 2 to 3 years. Each hair grows approximately 1 centimeter per month during this phase. About 90 percent of the hair on your scalp is growing at any one time. About 10 percent of the hair on your scalp, at any one time, is in a resting phase. After 3 to 4 months, the resting hair falls out and new hair starts to grow in its place.