Here is documentation about creating Cloud Watch
LogGroup via Cloud Formation. They said:
The number of days log events are kept in CloudWatch
Logs. When a log event expires, CloudWatch Logs automatically deletes
it. For valid values, see PutRetentionPolicy in the Amazon CloudWatch
Logs API Reference.
So if I create LogGroup without
RetentionInDays parameter will Cloud Watch keep those logs forever? Or what
RetentionInDays value they use by default?
By default, log data is stored in CloudWatch Logs indefinitely. However, you can configure how long to store log data in a log group. Any data older than the current retention setting is automatically deleted. You can change the log retention for each log group at any time.
Source :- http://docs.aws.amazon.com/AmazonCloudWatch/latest/logs/SettingLogRetention.html