public static IsolationLevel configuredIsolationLevel(ConsumerConfig config) {
        String s = config.getString(ConsumerConfig.ISOLATION_LEVEL_CONFIG).toUpperCase(Locale.ROOT);
        return IsolationLevel.valueOf(s);
    }
A fixed, large enough value will suffice for max.
clients/src/main/java/org/apache/kafka/clients/consumer/internals/ConsumerUtils.java