It depends. Some people routinely dust (every couple months, maybe??), others (like me) only dust when they see a problem. You can dust with a number of different things, the most popular seems to be Sevin, though I've never actually tried it myself. I dust with DE (diatomaceous earth) which has worked well for me and I don't plan on changing. A lot people don't like DE, they say it hasn't worked for them...so I've never been quite clear on that.
A good way to check for mites/lice is to hold her chicken on their tummy (keel bone) resting on your arm with your hand in between their legs, lift 'er up and check the vent. If you see a lot of creep crawlies scampering off on the skin - you know you have a problem. Though I don't now the details, lice are yellowish clear with three legs on each side? Mites look similar but are a lot smaller.
Also check the saddle/sickle and/or back feathers on your chickens, mites like to hide there. If you see yellowish clumps on the feathers those are lice egg sacks, if you see crawling small things running around on the skin/feathers (but more so on the skin) those are most likely mites.
Check under the wing, blow a bit on the downy fluff if you have too. The hackles are a good place to check, also. Oh! I almost forgot, check their...for lack of a better word 'butt fluff'. Kinda pick through there, scabs can also be seen on your chicken I'm not sure whether or not they are mites or lice, but are some sort of sign of bugs.
I wish you well!
WW