I heard so much great stuff about DE that we got some (Food Grade) and put some in with pine shavings on the coop floor. I'm the coop cleaner, so I pick up the poops each morning, but don't really disturb the litter. I had this weird thing where when I'd take a deep breath I'd start coughing, but I wasn't sick. And not just when I was in the coop. It lasted maybe a month until my husband put 2&2 together, decided it was the DE causing it. He went and cleaned the whole coop out, put in new litter (no DE this time), and lo and behold the breathing problem thing was gone like overnight.
And I knew ahead that you're not supposed to breathe in DE, and it wasn't like I was throwing it around and inhaling it or anything.
So Plan B is I want to make a sandbox (outside the coop) for them to dust bathe in, with dirt, sand, and some DE mixed in. Hopefully the girls will use it instead of making holes all around the yard for unsuspecting humans to trip in.
As to how often to use it, maybe refresh it every month or so? I think its "staying power" is long, unless it gets wet? (But I do not claim to be an expert on DE at all...)