The following is a very general rule for treatment with powdered sugar dusting, put together with my notes and help from Roy Cronkhite of Maine. Thanks Roy!
If there is no capped brood, treat only once, because all the mites are on the bees.
If there is capped brood, treat 3 times, about 5 to 7 days apart, to also affect mites which have recently emerged with bees.
If you see a severe infestation with capped brood ( you would notice your powered sugar is black with mites), you would need to take intensive action by dusting every 3 days for 3 weeks. This would insure capturing most of the mites in the capped brood and be knocking down your mite population.
We are dusting at the following 5 specific points of the year, you need to adjust to your part of the country.
1 - March or when bees flying -- warm enough to open brood boxes. No brood, dust once. Brood, dust 3 times.
2 - May -- after pulling spring honey
3 - July -- after pulling berry honey, this is our most critical time, allowing us to knock down the mite population before the Queen starts slowing down and drone brood ceases. This allows us to collect our thistle honey without worrying about the mites hurting our winter bees. Once again. If you can not get a mid-July dusting, the latest time before critial mite build-up is August 15th. At that point, your hive is producing its winter bees, the queen has slowed down and drone brood has ceased. That means mulitable mites in each cell containg working larvre. This is the scenario that produces bees with deformed wings and seriously impacts your hive
4 - September -- after pulling thistle honey.
5 - November -- queen is or has shutdown from laying, if no brood, dust only once.
You can dust in winter, whenever the bees are flying. In California we ususally dust once in January.