Guest Users governance - stale guest users

The "best practice" will vary greatly from one organization to the other, as some are very "open" in regards to guest users, other must allow their partners and customers, while organizations in certain industries will never even enable such a feature. Microsoft's own tool to manage guest user lifecycle is called Access Reviews (https://docs.microsoft.com/en-us/azure/active-directory/active-directory-azure-ad-controls-manage-guest-access-with-access-reviews), but unfortunately the license requirements are very prohibitive for any organization not already invested in AAD Premium P2.

PowerShell and the Graph API give you all the tools you need in order to manage Guest users, so you can build your custom solution around it, if the above is too costly.