Set the retention days on any AWS CloudWatch log group that isn't set yet

log-retain-manager-policy.json

{
    "Version": "2012-10-17",
    "Statement": [
        {
            "Sid": "SetLogRetain",
            "Effect": "Allow",
            "Action": [
                "logs:DescribeLogGroups",
                "ec2:DescribeRegions",
                "logs:PutRetentionPolicy"
            ],
            "Resource": "*"
        }
    ]
}

log-retain-manager.sh

#!/bin/bash

AWS="/usr/local/bin/aws --profile log-retain-mgr --no-paginate --output json"
JQ="/usr/local/bin/jq --raw-output"

for REGION in `$AWS --region us-east-1 ec2 describe-regions | $JQ '.Regions[].RegionName'`
do
  echo "Region $REGION"
  for GROUP in `$AWS --region "$REGION" logs describe-log-groups | $JQ '.logGroups[] | select (has("retentionInDays") | not).logGroupName'`
  do
    echo "  $REGION $GROUP"
    $AWS --region "$REGION" logs put-retention-policy --log-group-name "$GROUP" --retention-in-days 30
  done

done

If you don't know what jq is you are in for a welcome surprise

https://stedolan.github.io/jq/