“How long does a carbon facial take?”

It takes about 20 minutes to do a good full face carbon facial.
Mark B. Taylor, MD

Carbon facials, like IPL, and microdermabrasion are more marketing and bells and whistles than any supported by any hard science. So, the question would be better posed as not "how long...," but instead whether to waste the money. I don't know exactly what you are trying to treat, although my guess is acne related. You would be wiser to seek consultation with a board certified dermatologist to determine what is most appropriate for you.