Storey's Guide to Raising Ducks recommends that if signs of niacin deficiency are present, that the niacin should be given for ten weeks, if I recall correctly.
I think it may be difficult to find a more specific guideline because ducklings do not all react the same way with low niacin. For some reason or reasons, some ducklings seem fine, yet others with the same feed begin to have seizures or leg problems.
I would supplement, if I thought the feed may not have enough, or if any of my ducklings seemed to have any problems that could be niacin-deficiency related, until they were 12 weeks old, give or take. And I would continue to give poultry vitamins after that.