The answer is every 4 to 5 months. Eyebrow and eyelash hairs run on a much shorter growth cycle than scalp hair.
Don't mix this up with the daily-shedding number for scalp hair, which is a completely different stat (75 to 100 hairs a day is normal). Eyebrows and lashes aren't shedding a little each day so much as cycling through a full growth-and-replacement turnover roughly every 4 to 5 months.
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