How long I should leave part 1 cosmelan on? (Photo)

Cosmelan has an at home treatment which is not as strong as the MD preparation.  The MD preparation which is placed only in a doctor's office has hydoquinone and Retin-A compounded into the preparation.  The time left on is followed by guidelines from 5 or more hours depending on skin type.  Since this is a weaker preparation you should contact the company itself for guidelines.   

Cosmelan is almost always applied in a doctor's office because it has to be done a specific way..... You leave it on for 6-8 hours, but the physician and medical staff determine that. From your photos I don't see melasma which is what this treatment is for. I see other sun damage, freckles, and sites, but Cosmelan is not a treatment for those. "This answer has been solicited without seeing this patient and
cannot be held as true medical advice, but only opinion. Seek in-person
treatment with a trained medical professional for appropriate care."