:param significant_digits: int of the number of significant digits
            in the return
        :return: float of the time in seconds of the api call request

def duration(self, significant_digits=3):
        """
        :param significant_digits: int of the number of significant digits
            in the return
        :return: float of the time in seconds of the api call request
        """
        try:
            return round(
                self._timestamps['receive'] - self._timestamps['send'],
                significant_digits)
        except Exception as e:
            return None