Hair growth typically completes a cycle in 2 to 3 years. Each hair grows approximately 1 centimeter per month during this phase. About 90% of the hair on your scalp is growing at any one time. About 10% 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.