First, can I mention that it's not DUST mites that are a problem, it is other kinds of mites (of which there are a bazillion different kinds in the world), specifically the roost mite -- which is red, hides in crevices on objects in the coop during the day and sucks chickens' blood at night -- and the northern fowl mite -- which is sort of purpley brown and spends its whole life cycle on the chicken, sucking its blood -- and to a lesser extent the scaly leg mite -- which pries its way under scales on the legs and causes large misshapen areas. (There are other kinds of mites that affect chickens too, but those are the biggies). Lice are also a consideration for chickens; they behave more or less like the northern fowl mite but are pale bodied.
Anyhow, as to when to DO something about mites or worms:
Most people do not worm on a frequent regular basis unless there are specific reasons to think the chickens have worms, like you are seeing worms
or hens have unexplained diarrhea or they're on ground where you've had worm problems before. The difficulty with worming is that the only compound that kills 'most' kinds of worms -- there are a whole lotta different kinds of worms that a chicken can host -- is ivermectin which has no clearly defined withdrawal time for eating or hatching eggs but you should probably wait at least a month. Other wormers are available, some with better characterized official withdrawal times, BUT they only kill one or two kinds of worms. Thus, if you are worming broadly with ivermectin you may have to be throwing out eggs for a month or more, which most people don't want to do.
If your chickens are in an old coop/run and you suspect there may be a lotta parasite eggs in the ground, I suppose you could worm preemptively at 13-16 weeks before they start to lay. Otherwise though I wouldn't.
Some people do worm yearly, such as during a period when the chickens aren't laying much anyhow. Many do not, however, reserving it for if a problem is suspected.
For mites, if you provide a good dust bath, either manmade or just a dusty sandy place they scratch out in the run, you are a lot less likely to get mite problems in the first place. Some people add food grade DE do a dustbath to hopefully improve prevention; some (like me) put CONTROLLED non-crazy quantities of food grade DE in corners and cracks and crevices inside the coop and nestboxes and where the roost attaches.
You should get in the habit of inspecting the chickens for mites every week or so, though. Look around the vent, and make sure the leg scales look normal, and blow (with a puff of air, like blowing out a candle) the feathers apart on different parts of the chicken's bodies and look real, real fast for anything scurrying away.
If you see any mites, there are a lot more you didn't see, and it would be good to dust the chickens with something appropriate. DE is an ok preventative but IME it cannot deal with a serious infestation. Sevin dust is the most commonly used 'poultry dust' for lice and mites, but permethrin and rotenone compounds are also sold for this purpose. There are lots of ways to do it; I would recommend putting some in the foot of an old non-holey loosely-woven sock and using it as a powder puff. You want to be powdering the chickens' SKIN not feathers, except do the whole length of the tail feathers too. If you do it at night with a headlamp, the chickens will be sleepy and in the dark and will not protest.
Good luck, have fun,
Pat