How long this parameter value is valid.

        .. note: There is also an option when subscribing to get updated when
                 the parameter values expire.

        :type: :class:`~datetime.timedelta`

def validity_duration(self):
        """
        How long this parameter value is valid.

        .. note: There is also an option when subscribing to get updated when
                 the parameter values expire.

        :type: :class:`~datetime.timedelta`
        """
        if self._proto.HasField('expireMillis'):
            return timedelta(milliseconds=self._proto.expireMillis)
        return None