Under normal circumstances hair growth in each hair follicle occurs in a cycle. There are three main phases of the hair growth cycle; anagen, catagen and telogen with anagen further subdivided into proanagen, mesanagen and metanagen. Anagen is the active growth phase when hair fiber is produced. To keep things simple we are just going to focus on the main cycles.
Anagen is the longest phase with up to 90% of the follicles on your head in this active hair growth state at any given time. That means the majority of the hairs on your heat are actively growing. This phase lasts between 2 and 6 years. That's a long time for hair to grow.
The catagen phase is when your hair is lifting towards the scalp, still growing but moving up. This is a fast stage, lasting only a couple of weeks at the most and growing is slowed. A new follicle starts to form at the end of the catagen phase, in preparation to create a new hair strand. During the catagen phase the papilla (hair bulb) becomes separated and from this point, the hair can be pulled out when you manipulate your hair through combing, brushing or shampooing. At the end of the telogen phase, if the old hair strand has not already fallen out, it is pushed out by the new hair growing underneath.
Telogen hair follicles (resting phase) comprise up to 10% of your hair and lasts only 30 - 90 days.
All over your head, your hair is in different stages which is why you don't normally see large patches of hair shedding at the same time.
HAIR GROWTH RATE
The average rate of hair growth is around 0.35mm (1/4 inch) a day but this rate varies depending on the site and location of the hair. During anagen, your hairline may grow faster than your crown - or vice versa - and it's this phase that determines the length of your hair. The longer the anagen phase, the longer your hair grows.
The length of anagen is firstly determined by genetics, assuming you are healthy and there is nothing interfering with your optimum growth phase it will grow as long as it can. The length of the anagen growth phase for scalp hair is usually 3-6 years.
The first diagram on the left is showing regression of a mature anagen hair follicle. On entering catagen the dermal papilla condenses as the cells become inactive. With a lack of dermal papilla cell stimulation, the hair fiber and root sheaths stop growing. In telogen the dermal papilla can become isolated in the dermis and the hair fiber can easily be pulled out (by combing, shampooing, or brushing).
If the old fiber has not already fallen out it is pushed out by the new hair fiber growing underneath