I think this may answer my question. Unless the job has aged, the data will be retained by the SP on disk/tape.
What I was asking was, if a user deletes a lot of archived stubs from their mailbox, or if a lot of users are removed from Exchange, will that data still be retained in the SP, or will that space get purged.
Otherwise, if your archive has unlimited retention, anything that gets put in to the archive gets stored for ever. Even if the user no longer exists within the system.
I guess I can see why that is preferable, because if a user was deleted by accident, or a user deleted all their archive data from Outlook, and CV purged it from the storage policy, you'd not be able to restore it if needed as that data woudln't be in the backup data.
Does the above make sense?!