Anagen is the growth phase. This lasts for about 3 - 5 years, where you can observe your hair growing half an inch every month. Full-length hair from this phase is about 18 - 30 inches long. Studies show that this phase may also be affected by other factors. Asian hair, for example, has been found to have a longer anagen phase. Weather is also a factor; hair growth can be faster in summer than in winter.
Jimenez, J. J., Wikramanayake, T. C., Bergfeld, W., Hordinsky, M., Hickman, J. G., Hamblin, M. R., & Schachner, L. A. (2014). Efficacy and Safety of a Low-level Laser Device in the Treatment of Male and Female Pattern Hair Loss: A Multicenter, Randomized, Sham Device-controlled, Double-blind Study. American Journal of Clinical Dermatology, 15(2), 115–127.
Dupilumab is FDA approved for treating eczema aka atopic dermatitis and sold under the brand name Dupixent. As the story goes, a patient with alopecia totalis (a form of areata which leaves a person’s head completely bald) was being treated for eczema by the drug Dupixent. After 6 weeks the patient first began to notice progress in terms of hair growth and at 7 months she had noticeable pigmented hair growth on her scalp. Notably, the patient stopped taking the dupilumab for a period of time and noticed her growth subsided; when she began taking the drug again the hair improved once more. This provides another useful therapy option for patients seeking treatment for AA. One would imagine a topical version would be worthwhile to investigate.