A fungal scalp disease, an oily scalp, dry skin, stress, or conditions like seborrhoeic dermatitis can all cause dandruff that lasts two to three months. Regularly wash your hair and apply a Ketoconazole-containing anti-dandruff shampoo two to three times a week, leaving it on your scalp for five minutes before washing. If your scalp is greasy, try not to share combs, scratch it, or use too much hair oil. Stress can exacerbate dandruff, so eat a balanced diet, drink plenty of water, and get enough sleep. If you have severe itching, redness, pus, thick scaling, or accelerated hair loss, please consult a dermatologist.