I deworm with Wazine twice a year during my hens molt so it doesn't effect their egg laying. You shouldn't deworm chickens under 12 weeks of age, so I would recommend doing it once they start their first adult molt. Are you ok with chemicals or do you want a natural solution? I also do natural preventatives every couple weeks.
There's no reason to treat chickens for worms unless they have them! If you wonder if there's a problem, have a fecal run at your veterinarian's first. Many flocks don't ever have a problem with intestinal parasites, while others do. My birds have been clear for three years now. That could change, and then I'll deal with it!
I have never wormed mine with chemicals. I want to go as natural as possible and I figure back in the day, when my Grandparents had chickens, they didn't go to the store and buy chemical wormer, and their chickens were fine. I make sure they are healthy. I have a small flock and let them free range. And I use preventives like apple cider vinegar, garlic, and then every fall I mix up a treat that's supposed to be a natural wormer. I got the recipe for that and all of my other info from the book "Fresh Eggs Daily".
If you actually see worms (not maggots!) in very fresh poo. Some are too small to see though. Serious coccidia infections can cause bloody poo, but so can other things.
'Natural' products, as above, don't work, and those healthy birds are fine without using vinegar, etc.