Cloud-based Message Recall in Exchange Online

Sektorsync >>How long can a message be recalled after it has been received?

As per the FAQ above: Is there a time limit past which you can’t recall a message?
No, there is no time limit, just like the classic message recall feature. Providing tenant admins the ability to customize a time limit for their organization is under consideration for a future update.

Colin_Millins >>Will the recall report show which recipients had read the message?

Yes, it will show which recipients read the message for both recalled messages and messages that failed to recall. You can sort the Details column in the report's recipient table to more easily identify those that read the message.

Sebjogro >>it is not possible to inform the recipients of read AND unread messages that a message has been called back?

That's correct, at this time the recipient does not receive any notification that the message was recalled; true for both read and unread messages. However, if for legal reasons you especially want that for read messages but it's not necessary for unread messages you can disable the recalling of read messages so that unread messages still get recalled but read messages don't. You can do this via the  checkbox in the Exchange Admin Center > Settings > Mail flow section, or using the Remote PowerShell cmdlet: Set-OrganizationConfig -RecallReadMessagesEnabled $false.