initially the investigation would go back 12 months. If the OR chooses to from that evidence he can go back as far as he likes. There's no set limits as such but the statute of limitations would affect anything further back than 6 years. Also the absence of a credit history beyond 6 years would affect the ability of the OR to go back any further.
So the OR is not restricted but it would be very difficult for him to evidence anything beyond 6 years historically.