When does the soul leave the body, when the heart stops or the brain stops?

You have to actually be dead, and we are having difficulty defining death these days because there can still be brain activity. I would say that once the brain ceases its activity, that is the best definition of death we have so far. When the heart stops, after about six minutes the brain has no oxygen and goes into another state, though we do not know for certain that six minutes is the absolute limit, because some people have come back from having no brain activity, which is what makes it difficult. I once took a whole seminar on how to define death, and with modern medical techniques the definitions are getting more confused rather than more specific. But no one has actually lived beyond their death certificate; the coroner is usually cautious and pronounces death when both the heart and the brain have stopped. Another way to put it is that you cannot have life without the soul present, so if the soul is gone, that itself is the definition of death, though the difficulty is that you cannot see the soul leave the body.