Hair on the scalp grows about .3 to .4 mm/day or about 6 inches per year. Human hair growth and shedding is random and not seasonal or cyclical. At any given time, a random number of hairs will be in one of three stages of growth and shedding.
Anagen (Active Growth Phase) - Hair grows thick - It is active phase of the hair. This is the longer phase of hair growth for 2 to 6 yrs. During this phase the hair grows about 1 cm every 28 days.
Some people have difficulty growing their hair beyond a certain length because they have a short active phase of growth. On the other hand, people with very long hair have a long active phase of growth. The hair on the arms, legs, eyelashes, and eyebrows have a very short active growth phase of about 30 to 45 days, explaining why they are so much shorter than scalp hair.
Catagen (Not Active Phase) - Hair is not in blood supply - This is a transitional stage and about 3% of all hairs are in this phase at any time. This phase lasts for about 2 to 3 weeks.
Telogen (Resting Phase) - Growth stops completely - In this phase the detached hair pushed by the new growing hair. This process is known as shedding of hair. This phase lasts for about 100 days for hairs on the scalp and longer for hairs on the eyebrow, eyelash, arm, and leg. About 25 to 100 telogen hair are shed normally each day.
When old hair falls out and new hair grows is the sign of early anagen stage.
IMPORTANT POINTS TO CONSIDER WHILE USING HAIR GROWTH PRODUCTS
A hair growth product should be applied minimum of 90 days (3 months) to see the actual growth.
It's normal to have excess hair fall for the initial 1-2 weeks of hair growth product application. Hair growth product application triggers hair growth stage change from Telogen to early Anagen which results in excess hair fall.