That is all subjective as it depends on your policies when this happens.  Do you allow users to take their email with them?  Does Security & Compliance require it be kept for X days?

It is one of those “just depends” type answers based on your company policies. 

I can’t really speak for backups, but I would assume it would follow what I do with the actual mailbox.  Many of my clients just delete the mailboxes so I wouldn’t be concerned with the backups sticking around.  If it’s something that they need to keep for archiving or future access by others/replacements, then I convert the mailbox to Shared, unlicense and assign permissions for whomever needs access if anyone, but of course I would still want the shared mailboxes backed up.  It’s really a case-by-case of the business need to retain that data, or not.

Like others have said, it depends. That said, I’ve dealt with a lot of clients and I’ve seen most leave the mailbox for 1 month, then delete, so it keeps noise down on the amount of recovery requests.

After the month, most organisations want to keep the data for a period of time after the users have left, typically either for the full backup retention (as there could be regulatory or legal/data hold requirements), or say a 50% retention period for ex-employees, typically for organisations that are concerned with shorter term scenarios such as ongoing projects etc.

You’ll only be carrying the license of the ex-employee for 31 days after the mailbox is deleted from a VB365 perspective, so that shouldn’t be too painful for most organisations, especially as VB365 offers overages such as 10 licenses or 10% extra licenses, whichever is greater.

If you’re happy to leave the data for the retention policies, no further action is required, otherwise you can leverage the Remove-VBOEntityData cmdlet to purge sooner than the retention policy.

For more licensing details this link will help: https://forums.veeam.com/post367734.html