Hair grows in three distinct cycles: anagen, catagen and telogen. At any one time approximately 90% of hair follicles should be in anagen phase. Anagen phase (or growth phase) should last between 2 and 8 years. At a growth rate of 0.5 inches per month, it should take around 3 years to grow your hair 18 inches long.
Only 10% of the hair on the scalp should be in telogen phase. In telogen stage, the hair has transitioned out of anagen stage and will rest for 2-3 weeks before shedding. Hair loss will start to become evident when more than 30% of the hair is in telogen stage. With an increased number of hairs in telogen phase, hair shedding will increase and appear not to grow any longer. To regrow hair, the anagen / telogen balance must be restored.