How long will it take (if ever) for this lump of extra skin in the conchal bowl to flatten post otoplasty?

It is difficult to assess your problem without pictures.  Typically, with resection of excess conchal cartilage one sees the skin redrape without complication.  The conchal bowl is usually packed with vaseline/iodine impregnated gauzed to bolster the area and keep the skin flap against the conchal cartilage in the immediate postoperative period.  If you surgeon has already ruled out hematoma,  and infection, I would suggest you allow for at least 6 months recovery to see if it flattens on its own.  If it does not, it can be revised by elevating the skin and redraping it again.  Good Luck

It would nice to see a picture but there is always something that can be done about excess skin. First and foremost, make sure the problem doesn't go away with time alone. Consult your plastic surgeon and trust his advice. Be patient.Best wishes