However, hair can't be infinitely long. Under normal circumstances, when the hair grows to 50-60 cm, it will fall off and regenerate new hair. 2. Growth cycle Hair can be divided into three stages: growth period, transition period and dormancy period. The growth period of hair is 2-7 years, with an average of 4 years. A few heads occur for a long time up to 15-20 years, and the longest can reach 25 years. The migration cycle of hair is 2-4 weeks, which is the shortest compared with other parts of hair. The dormancy period of hair is 3-4 months.