Birds that have been very badly affected by worms may need a second treatment 3 weeks later, but normally every 4-6 months is adequate for hens kept in clean conditions. Hens with access to pasture which has had poultry on it for some time are more at risk because they will eat earthworms, which may have picked up chicken worm eggs from droppings in the soil and the hens can ingest these when eating the earthworms, thus completing the cycle as the worm eggs then hatch in the hen's gut. So if this is a risk, every 4 months is better. Also, chicken worms are more active in the warmer months, so if you treat the flock in the Autumn they should be OK until the Spring.
You can use Diatom in their dustbath, but possibly the most effective treatment if you actually see any lice eggs is to use louse powder, which I think is diatom plus an insecticide. A somewhat different kind is sold as redmite powder, under brand names such as Smite. It smells nice and can be used in the coop though many people recommend making it up into a slurry and painting it on, to reduce the risk of breathing in toxic dust.
The main thing to remember about ACV is to get the unrefined sort, as sold in agricultural stores etc, and not the poncy refined kind for humans from Waitrose which is supposedly less effective. However, use with caution - many of us on here don't use it at all, at least not routinely on healthy hens. It certainly won't kill worms, and like Vermex, any other alleged benefits are unproven. I once experimented by putting out two drinkers, one with a weak solution of ACV as directed on the bottle, and one with plain pure water. The flock ignored the ACV altogether and just drank the pure water. I concluded that forcing them to drink ACV might make them drink less than they would otherwise do, which could be dangerous in hot weather and also affect egg production. Also they clearly didn't like it. So I poured it down the drain and notched it up to experience.
So just to reiterate what I said the other day - healthy hens do not need any dietary supplements and they are in most cases a waste of money. Just my opinion, though.