Senna - that's great, good luck.
Lucylou - if you continued for a couple of days, it would be more than advised. The nil egg withdrawal period is based on the chemical being lower than a certain legal level in the eggs - so I guess you'd probably exceed that by continuing.
In the flubenvet literature, there are some graphs that show worm numbers against time - there is a huge decrease after just 2 days - then it tails off. If it were me, I wouldn't bother worming again, unless you know you have a bad infestation or they have been kept on the same bit of ground for years.
Remember, there will usually be a small background level of worms in most flocks- and the birds build up resistance to them.
Hope this helps.