The dosage will vary accordingly to age, currently, we go, 1/4ml for ducklings that are between one to seven days old, 1/2ml for ducklings that are one week to two weeks, anything above that will be 1ml, this is once a day, although if you have a hard time getting the complex into the duckling you can split the dosage in half and give some in the morning and evening. The effects of giving too much vitamin B are somewhat limited, it does seem, however, that you would need to give a large amount to the duck for it to have a damaging effect.
This is a quote from John Metzer
"
We have been asked if it is possible to overdose on niacin. Because of how small the amount is, it is easy to think you can give your birds too much. The answer is that you would need a lot to overdose. We don't have an exact amount to point towards, but humans can overdose on niacin and that can cause bowel problems. If your birds start having digestive issues, then it's a possibility, but is unlikely."
We recommend for novice duck keepers to give them the complex over treats, to reduce the risk of aspiration if given orally with a syringe. Duck seems to enjoy mealworms, peas, lettuce, tomatoes the most, it can also be mixed into a wet soupy feed. The reason we recommend not giving it in water is most of the time ducks spill around half of the water, so it makes it hard to calculate the dosage, and birds that are in poor condition may not be able to get around to drink often. If your bird is not able to get around or is so debilitated, you may need to administer orally with a syringe/eyedropper, the easiest and safest way I have found is to drop the B complex onto the side of their bill, the sensation of the water droplets often will make them drink the liquid, another technique is to dip their bill for a short second in some of the liquid. If this doesn't work you may need to stick the syringe in their mouth, I will provide an article on the bottom on how to do that correctly.
https://www.backyardchickens.com/threads/niacin-deficiency-in-waterfowl.1367557/