How would a 19 year old male with relapsed all leukemia progress if he decides to stop treatment entirely? How long would he last?

ALL. Acute lymphoblastic leukemia is treatable on relapse however without treatment often is rapidly fatal. There are many excellent centers that treat all and adolescents and young adults. I would strongly urge him to seek a second opinion at one of those centers.
It's impossible.... To know. I've had patients who i thought would succumb to their disease quickly survive for over a year without treatment while other patients who appeared well and i thought would survive for a while died quickly. The leukemia is part of the picture, but risks for life threatening infection, acute and chronic end organ failure, etc. Further complicate the clinical picture.