I am 24 days post op, SMAS lower and neck lifts.​ How long until scabs fell off incision lines?

Than you for your question.  For the most part  the majority of the patient don’t have scabs after the second week, but if the are scabs I prefer not apply anything on them before the second week after  I would consider light petrolatum   twice a day.

It's quite common to have some scabbing on surgical incisions; this is often a combination of old blood and dry skin.  More rarely, scabbing can represent skin that has necrosed or essentially died, usually because of inadequate blood supply.  It's difficult to give a tailored answer to your specific question without the benefit of photos or an in-person exam.  That said, keeping an incision (and scabs) moist with an ointment like Neosporin or Aquaphor usually speeds healing and the resolution of scabbing; I commonly include this in my post-operative facelifting and necklifting instructions.  Please check in with your surgeon for further guidance, and best of luck in your recovery.