Cloud Watch logs for long term retention

It would not make sense to use CloudWatch logs for long term retention. As you say, best practice is that if log data is needed to be retained for historical reasons that it is exported to S3 and then put in Glacier via S3 lifecycle rules leading to long term cold storage pricing from $0.004 / GB.

Typically, long term logging data is only required for compliance / legal reasons and is not something that would require frequent access so why pay more in CloudWatch?

In terms of maximum size, I do not believe there is an upper limit (or rather one which someone would hit) By default, CloudWatch Logs will store your log data indefinitely. The documentation implies that there is no upper limit to how much data can be stored:

Log data can be stored and accessed indefinitely in highly durable, low-cost storage so you don’t have to worry about filling up hard drives.

Other than 5GB of storage for the free tier, there is no mention of an upper limit for log storage in the CloudWatch logs limits documentation

Best regards