There is no way to know, either on BA or CX.

If the agent told you that the bag was loaded on a specific LHR-NRT flight arriving later in the day, then it will likely be on that flight. If the agent told you something to keep you calm, then it may or may not arrive.

There are no "usuals" in the baggage delay business.

The real issue is that CX must have good contact information for you in Tokyo so that the bag can be delivered. A hotel, a place of business or someone around to receive it.