How long can the human heart survive without oxygen?

as such its very vague. because it depends on part to part if the upper limb. loss of blood supply to any of the body organs can lead to avascular necrosis i.e death of the tissue due to loss of blood supply. some parts are very sensitive to this as necrose immidiately after loss of blood supply whereas some parts can resist the stress and necrose after some time. but its generally said that that the avascularized part's blood supply should be fixed within 5 to 8 hours(for muscles generally) (but brain dies if it doesn't get fresh blood supply within 15 minutes as it can utilize only glucose)