I've tried several situations for long_press;
    self.marionette.find_element(*self.<app locator>).long_press()
    self.marionette.long_press(self.marionette.find_element(*self.<app locator>))
    also
    self.marionette.long_press(self.marionette.find_element(*self.<app locator>), 3000 to 6000) - because the default is 2000