The lice eggs will hatch every 10 days or so, and those lice will be too young to breed yet. So theoretically, dusting or spraying every 10 days should get all of the lice and the newly hatched ones before they lay new clumps of eggs. A couple of good articles to read on lice treatment are TheChickenChick.com, and the one from UCDavis if you Google "mites and lice in chickens."