def running_time(self):
        """
        For how long was the job running?
        :return: Running time, seconds
        :rtype: Optional[float]
        """
        if not self.is_done():
            raise ValueError("Cannot get running time for a program that isn't completed.")
        try:
            running_time = float(self._raw['running_time'].split()[0])
        except (ValueError, KeyError, IndexError):
            raise UnknownApiError(str(self._raw))
        return running_time