How long does it take a chemical burn on the lips from teeth whitening to heal? it keeps flaring after more than a month and spreading on my face

Call PCP: Healing does depend on the amount of area involved in the burn. From the sounds of it, you need to consult your pcp or maybe a plastic surgeon for appropriate care.

3-Day Rule: The wonderful thing about the healing powers of tissues of the mouth is that healing is generally rapid. Any lingering deficit should be brought to the attention of your dentist asap! i give my patients my "3-day rule" that says--"if any change in the mouth lasts more than 3 days or happens more than 3 times, you need to see your dentist--because it's likely a bigger problem than you think!".

7-10 days: I would be concerned for any wound to be active for over a month. Normally healing is 7-10 days. Non healing after 2 weeks is very concerning. You may have a systemic condition that delays healing. I would see a physician to rule out medical condition. And discuss the chemical burn with your dentist. 1 hour whitening is the least effective way to whiten teeth, with the most risk and side effects.

See your physician : Burns from teeth whitening are usually gone within 2-3 days. If this is spreading it's not a result of the peroxide and you should see your doctor.