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

Create a new IAM user, with CLI access, get the access key and secret and load into a AWS CLI profile log-retain-mgr
Give the IAM user the policy seen above
Run the script, it will cycle through all AWS regions, find any CloudWatch Log Groups that have no retention setting, and put a retention days of 30 on them

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

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