Withdrawal time for off label wormers is usually 10 to 14 days after last dosage.
Ivermectin is a systemic wormer, in the same category as the ant poison Avermectin, meaning it gets into the blood stream to attack the neural system of the parasite through the blood pathway. Some individuals have sensitivity to it, due to a particular genetic mutation, that allows Ivermectin to cross the blood brain barrier. (Collies and Shelties in particular).
Knowing that, it is likely to get into the eggs and it could conceivably cause issues with someone who is sensitive. If that concerns you, or if you have individuals in the household that have chemical sensitivities, you may wish to withdraw eggs.
On the other hand, WHO champions Ivermectin use in third world countries for protecting children from dreaded river parasites...so it is a human dewormer as well.
I archived this sometime ago in regards to parasite control:
http://www.meetup.com/AustinBackyardPoultry/messages/boards/thread/23965462
Having said all that, Ivermectin is indeed off label for use in poultry, which means you use it at your own risk and could never sell the eggs from those hens... ever.
I prefer to stay on label, as I do sell my eggs, and I have concern with Ivermectin because of the known potential for chemical sensitivity in certain individuals.
I have read many people use it for birds, and if you have a tough infestation, or a particular parasite such as gape worm or tape worm, which the regular poultry wormers generally won't touch, it may be something to consider if it is for your flock and family use only.
However, if this is for general use of typical parasites, it may be worthwhile to find something that is on label and sanctioned for poultry used for egg laying.
Unfortunately there is only one current FDA approved poultry wormer in the US, so your choices are limited.... Hygromycin B, but you can get it in Rooster Booster Triple Action Multi Wormer or in Durvet's Strike III at your local feed store. It is in pellet form that you add to feed. Commercial industry keeps it continuous feed, but most backyard farmers use it generally for 2 weeks continuously, 4 to 6 if heavy infestation, repeated seasonally.
There is no withdrawal period with Hygromycin B as it is a mycin type wormer that stays in the gut starving the worms (it interferes with the metabolism in the worm) and does not enter into the blood stream so therefore neither the eggs (per research studies).
FYI to use as you see fit.
LofMc