Hair growth cycles depend on the site

Hair in different body regions has different active growth and full-cycle durations. The scalp growth phase usually lasts years—about 2–6 years or longer—while eyebrow and eyelash growth phases are much shorter, from weeks to a few months. Axillary hair cycles are also shorter and variable. Thus, “3–4 months” is not a universal lifespan for each hair: follicles cycle asynchronously. [1–3]