As you know, administrators can allow end-users to share content with external users (guests).

That said there was no way to easily get these access to expire or being revoked.

Well good news, administrators can now automatically revoke guest access to SharePoint Online or OneDrive after a number of days.

To enable this, logon to your SharePoint administration portal (https://<your tenant>-admin.sharepoint.com) and access the Policies\Sharing blade to enable the Guest access to a site or OneDrive will expire automatically after this many days