Zero. This has happened to me several times on several different airlines (not just OW), and while the airlines are invariably apologetic there is nothing that can be done about it unless one of the confirmed pax no-shows. Unfortunately this is one of the negative aspects of having split PNRs. I suspect this situation has gotten worse in the past year with some persons having software that hammers the reservation system with thousands of requests waiting for these windows to open up -- so it is guaranteed that they will snag the seat when your agent has to shuffle things.