llaumgui soft deleted messages are never permanently deleted automatically. Permanent deletion can only be performed by mods/admins with the "Delete posts forever" permission.
If a user soft-deletes their own post, they can still restore it anytime if they have the edit post permission and the discussion isn't locked. If a moderator deletes a user's post, they cannot restore it.
Extensions could provide automatic deletion or modify the way the hide/restore permission works.
