rat lice

So I've been applying food-grade diatomaceous earth to my rats for a little over 30 days now. I heard that I should keep doing this for 8 weeks on a different animal. I was wondering if anyone knew how long I was supposed to keep applying this. Like what is the full life cycle of rat lice?

Note to those who might not know DE means diatomaceous earth. And also, I'm using food grade DE. So my rats have been having a lice problem and I've already applied it to them twice within two weeks. It's always very hard and stressful to them. I was wondering if anyone had tips on how to apply...