Why there is a 4-6 months wait for new hair growth:
There is a 4-6 month wait before new hair grows after treatment because the hair follicles have to shift from a resting stage to a growing stage. Then the new hairs have to grow out long enough so that they are noticeable. All of this takes some time.
Cycle of Growth:
Hair grows from hair follicles, which are miniature organs in the skin. Each individual hair follicle grows a single hair in a cycle consisting of a long period of growth (about 5 years) followed by a relatively short period of rest (about 3-4 months). After each rest period, the hair is shed, and a new hair begins to grow, and the growth cycle starts again.
At the end of the growth period, the follicle prepares for a rest. Hair growth for that particular follicle stops. The transition to the resting period is called the catagen stage. This stage usually lasts about a month or so, and during catagen the lower portions of the follicle collapses, and the grip on the hair becomes loose, which allows the hair shaft to be shed.
The resting period, called the telogen stage, lasts for another 3-4 months, and sometimes a big longer. Hair follicles that have been transplanted usually make a transition from growing hair to no growth, and then enter the telogen phase, shed their hair, and then after 4-6 months start growing new hairs. New hair growth resulting from pharmaceutical treatments such as minoxidil and finasteride comes from "waking up" resting hair follicles. It takes some time before these "resting" follicles shift to the growing phase.
The hair growth period of a hair follicle is called the anagen stage. The anagen stage for scalp hair follicles last about 5 years. The new hairs grow about 1/2 inch per month.