We see a lot of B3 deficient ducklings on here, especially heavy-weight breeds like Pekins since they grow so fast and put a lot of weight in a short matter of time, they seem to have a higher niacin intake than other breeds. It partially depends how long they've had it before it's corrected, for example, some day-olds come down with it and we've seen a complete turn around in a matter of hours, An older duck around two months may take a few days/weeks to be totally better.
To save you another trip make sure to get a syringe with a needle so you can get the fluids out of the bottle.