There is a vet on a chicken Facebook page that says withdrawal is 8 weeks.
Not disputing your post however I find it odd that for the minute amount injected and that ivermectin is used to treat some types of parasitic infections in humans that there is any withdrawal at all. IIRC the recommendation for withdrawal hinges on the fact that ivermectin is off label use in poultry. As off label use, there cant be a documented withdrawal period.