I admit that I was never much of a worming chicken keeper, though everyone was wormed about once a year. That was when they were all free ranging much more than they are now. Penned chickens need worming more regularly, IMO, than ones who can access natural wormers out on range while foraging (don't ask me what they are, though, can't remember where that list is, again with the memory!). I would probably recommend worming twice a year. You then have to do a second one two weeks later, if you want to be thorough, to get worms that hatch from those internal eggs the meds don't get. I've usually skipped the second phase, but this time, I am going to do it.
I am announcing that today, I bought eggs for only the
2nd time since owing chickens. They look anemic and will probably taste the same (not used to seeing stark white eggs!). Blech! But, I have to worm them and they are not laying much anyway. Got 2 LF an 1 bantam egg today, which added to every day's take, will be fed back to the masses until the second worming. Wormers have different withdrawal periods, but those are just estimates and some folks don't even do them. I usually wait over a week, sometimes two weeks, before I eat their eggs, however, since I'm re-worming in two weeks, it will be awhile.
If you look up any of dawg53's posts, he is considered quite the expert on worming around here, Angela.