Based on the product information, one Garnier Pure Charcoal Black Serum Mask supplies about one week’s worth of serum, so using it roughly once a week (about 4 times per month) is reasonable for best effect.

🩺 Recommendation

• Standard use: 1 mask per week ≈ 4 times/month (matches the product claim of “1 week of serum” per mask).
• If you want extra benefit: up to 2 times/week (≤8/month) only if your skin tolerates it and you don’t develop irritation.
• Lower-frequency option: 1–2 times/month for occasional maintenance or if you have very dry/fragile skin.

🏥 How to use

• Apply to a clean, dry face with protective film removed; leave 15 minutes.
• Gently massage leftover serum into skin; do not rinse.
• Do a patch test before full-face use and store in a cool, dry place away from sunlight.

⚠️ Warnings

• Stop use and see a doctor if you develop persistent redness, swelling, burning, or blistering.
• Avoid contact with eyes and do not apply to broken or inflamed skin.
• Discontinue if you notice allergic signs (hives, throat tightness) and seek urgent care if breathing is affected.

🩺 Next step

• For personalized advice (if you have acne-prone, sensitive, or reactive skin), consult a dermatologist.
  If you want, I can explain the ingredients or suggest a weekly routine tailored to your skin type.