How Often are Marketplace OS Images Updated

It depends on each marketplace seller on how frequently they updates and publishes an updated OS image, or when updates are made available to AWS. For example, Kurians Linux operating systems AMIs are patched and are uptodate on the day of the release, and the updates are published every quarter.

https://aws.amazon.com/marketplace/pp/prodview-qmkw5yzdbz57q

AWS provides updated, fully-patched Windows AMIs within five business days of Microsoft's patch Tuesday (the second Tuesday of each month).

https://docs.aws.amazon.com/AWSEC2/latest/WindowsGuide/aws-windows-ami.html#ami-patches-security-ID

And, in case of Red Hat, all instances of Red Hat Enterprise Linux on Amazon EC2 will receive updates at the same time that updates are made available from Red Hat. Please see below faq:

https://aws.amazon.com/partners/redhat/faqs/

Hope this helps!