Eyebrow hair have a very short cycle compared to head hair. For eyebrows, the whole cycle is completed in around 4 months, while it takes the scalp 3–4 years to finish. This is why eyebrow hairs have a much shorter maximum length than the hair on your head.
Eyebrow hair that is plucked or threaded begins re-growing after about two or three months. Plucking involves using tweezers or waxing to remove the hair by its root from the hair follicle, which causes temporary damage. The hair follicle then needs time to reboot its interrupted growth pattern. Over the course of a person’s life, the speed of the brow hair re-growth cycle slows down from an average of 56 days for eyebrow hair to regrow fully during early adulthood to about 73 days for a person at retirement age.
Consistent eyebrow removal can eventually lead to diminished growth, thinning eyebrows and dormant follicles, especially after the age of 42. In addition, eyebrow density varies from person to person because of the genes that gave them their follicles and dictated the length of their growth cycles.