My PS says it's a Dermatitis rash. I forgot to ask when it will start to go away? (Photo)

I encourage you to contact your surgeon to discuss your questions.  It may be if it is a contact rash that a different support bra may be recommended.  The rash will start to improve once the irritant causing the rash is removed and treatment begun.

including the garment, detergent used to clean your clothes, medications, and from products used for wound care or scar management.  So you have to do some homework and try to eliminate the possible 'allergen's to figure out which one is the culprit.  Your medications will help with resolution of your symptoms but if you don't eliminate the culprit, your rash can persist.