Thanks everyone for the inputs. To follow up as a data point for anyone else, the travel credit ended up reappearing in my account late last night (early this morning), so a little over 2 days after cancellation. Unfortunately it was just a bit too late to do what I wanted to do -- I had another reservation that I was going to cancel and rebook using the credit, but the "24-hour" refund ability expired on that a couple hours before (~26 hours after booking). Oh well, the returned credit has the same 12/31/23 date, so I should be able to use it for another trip.

I did call yesterday afternoon, and the agent saw that the ticket was submitted to the refunds department but said there was nothing she could to do expedite or manually process it, so I didn't push it. I don't think that affected the time.

Additional followup -- semi-related to this the fares for the flights were jumping around the last few days. I wanted to switch to a later redeye flight that came down to the same price as what I booked, but didn't as it would have been additional cost since you lose the Break from Business discount when changing (and as above it was just a bit too late to cancel&rrebook). Later this morning the better flight's regular price came down even lower than what we paid, so I did a change. THAT credit (for ~$25 each) appeared in my account instantly, and has the same 12/31/23 expiration.

I did a test booking and both (these are listed as Future Flight Credits) can be applied to the same new booking at the payment screen.

So it appears that when possible it's better to change a flight that was purchased with an FFC rather than cancel&rebook; this aligns with WineCountryUA's advice above. In our case it was complicated because I booked 3 of us together (only one with a credit) and would have had to do some splitting to do the right thing.