Your questions:
Thank you, Amiga, for the input. In your comment you say "based on her diet." I'm curious what aspect of her diet could cause a niacin deficiency. Is it that I'm feeding chick starter? The spinach treats? I'm asking because I have 10 more babies, and if it is possible, I would like to prevent any future niacin deficiencies. How would my duckling have become deficient in the first place? Chick starter should be nutrient complete for chickens and is widely recommended as duck feed. Does it lack sufficient niacin? If so, why is it recommended? Also, when feeding a niacin supplement (in my case, brewers yeast), how often, how much and for how long does the supplement need to be fed?
Chick starter does not have enough niacin for ducklings, which require three times the niacin that chickens do (per Holderread in Storey's Guide). There are articles that report no problem with ducklings on chick starter. My gut is that some ducklings manage okay (may have internal problems not visible, or get enough from bugs, who knows), while others are more sensitive. I have a duck that has trouble getting enough calcium, for example, and the rest do fine.
Spinach is said to be a problem for calcium uptake - I have not heard it interfering with niacin.
Yes, chick starter is widely recommended as duck feed. What can I say? It probably goes back to a wide spectrum of sensitivity in ducks to niacin deficiency, and whether or not they get niacin from bugs and worms or not.
Please check with a search of the forum, but I recall those using brewer's yeast sprinkling it on the food. I don't recall the exact amount - some don't seem to measure, just sprinkle. And it seems to be a developmental need, so I'd say till they're seven months old to be safe.