Do you know your chickens have capillary worms?
With any wormer you need to be sure to repeat in 2 weeks. Levamisole does have a 2 week egg withdrawal period, so that means you have a month of no eggs. Some people do still eat the eggs, but that's up to you. If you have more than one flock, you can do one flock first, then the other a month later so that you always have eggs. How often you worm is up to you, and depends on several factors. If your chickens are free-ranging and therefore potentially eating many sources of worms, then you might worm every 6 months or more. That said, if you use levamisole and follow the egg withdrawal, you'll spend way too much time not eating eggs. I have levamisole, but I haven't used it because of the withdrawal period.
All things considered, chickens that are free-ranging can become infected with worms the day after their worm medicine stops working, or not for 6 months or more. You might not ever even see evidence of infection, eg. worms in their poop or seriously decreased egg production. For me, I went a year and a half without worming, and I wormed because I figured I should. But, I've never seen any direct evidence of worms even though they almost certainly have some. Interestingly, people from different countries have widely different views on this. I'm currently in South America where no one worms their chickens, and if you read the Aussie forums they rarely worm either, suggesting that chickens tolerate low-level infections of worms quite well, and some say that frequent worming is harder on the chickens than the worms themselves. Of course, there's always a tipping point, and a chicken can become so heavily loaded with worms that it weakens and dies. From a biological standpoint though, this isn't the goal of the parasite, and in cases like this there are probably other factors weakening the chicken.
