I buy it by the 50# bag. My run is 8x40. I use two bags a year. After cleaning the run, I use a big old 2 qt yogurt container to dip in a cat litter bucket full of DE and put down a generous layer over the entire run, more where the girls dust bathe. The girls will mix it up.
I thought it was needed absolutely. I am putting my coop where there have never been any animals. Its up in a cinderblock planter next to my pool. If I don't have to go to the expense of it I would rather not! Thanks for all the advice!! Everyone here is so wonderful! I've never used it before, but lived in a much more rural area then. Now I'm in the city 7 miles east of downtown Los Angeles.
Really, really, you don't. Stall Dri and some similar products have DE in them and also dry up the litter. Or you can simply add pine shavings (or whatever) to help control poop and odor as needed, and / or change as needed. Agricultural lime, pelletized is much easier to handle, lots cheaper, and also helps control odor, moisture and flies, better than DE does in my opinion, and I have used both.
I have a big bag of DE in my shed I originally bought for worm control in the garden, but used little when I discovered how irritating it is in the lungs, as it "clouds up" everywhere. I use it once in a while but will probably never use it all.