def expires(self):
        """Union[datetime.datetime, None]: The datetime when this model
        expires. If not present, the model will persist indefinitely. Expired
        models will be deleted and their storage reclaimed.
        """
        value = self._properties.get("expirationTime")
        if value is not None:
            # value will be in milliseconds.
            return google.cloud._helpers._datetime_from_microseconds(
                1000.0 * float(value)
            )