15 Days Since Brachioplasty. Wound Separation in Underarms. How Long to Repair Itself?

I'm sorry to hear about the complication you have experienced after arm lifting surgery. Depending on the size of the wound, it may take a few weeks before it heals completely. In the meantime, I would suggest continued close follow-up with your plastic surgeon who is in the best position to advise you more precisely.

Best wishes; hopefully you will be very please with the long-term outcome of the procedure performed.

It's not unusual for patients to have wound healing problems following arm lift surgery. The problem typically occurs where the vertical incision meets the transverse incision. This location is at risk for wound separation for a variety of reasons. These include poor blood supply to the skin flaps and high levels of bacteria that occur in the axilla. Both of these factors can contribute significantly to wound breakdown in this area.Once breakdown has occurred, wounds tend to heal by secondary intention. Tissue tends to form a layer of granulation and wounds contract. The skin grows over the top of the granulation tissue. In the initial phases of healing, there may be an intense inflammatory response that can make the wound look red and angry. Depending upon the size of the wound this process may take 4 to 6 weeks to reach completion.For these reasons, these wounds should be monitored closely. If infection appears, antibiotics should be started immediately. In the interim, local wound care should be continued with dressing changes. When this approach is taken, wounds tend to heal nicely. Rarely, scar revision may be necessary as a secondary procedure. If you have questions or concerns about how your wound is healing, it's important that you discuss them with your plastic surgeon. Your surgeon should be able to address your concerns and alleviate your anxiety.

Unfortunately the arm pit is a difficult area to heal because of frequent motion and high normal bacterial counts, sweating, etc. Arm lifts therefore occasionally have problems like this, especially in the axilla (arm pit). It can take weeks to completely heal, sometimes months.
The most common reason would be absorbable sutures that are reactive and being pushed out  through the the skin but an infection could also be the cause.
The good news is that your arm lift will heal. It just needs more time.
See your board certified plastic surgeon to be checked.