If this is hypopigmentation, is it permanent? How long before it will fade back to normal? (photo)

Its best you stay out of the sun and apply spf daily to prevent the area from further darkening, this may take a few weeks but should lighten

Dear writer, what you appear to have are burned areas of skin where the home use treatment was applied. They are typically superficial and respond well to steroid creams ('cortizone 1') which will reduce the initial redness and inflammation. A topical antibiotic ointment may be advisable as well (polysporin or triple antibiotic). You may apply both, together, twice daily for 10 to 14 days. Sun avoidance is mandatory to prevent darkening of these areas. If excess pigmentation still develops there, then you may need bleaching creams prescribed by your dermatologist or plastic surgeon. Best wishes for a complete recovery, Dr. Bowes from Miami

This is called post inflammatory hyperpigmentation and will likely resolve over weeks to months. A dermatologist may prescribe a topical cream that will help the brown areas resolve faster-- but there will still be a waiting period before your legs look back to normal. Try to baby your skin-- gentle cleansers, moisturizers, and no sun!