Actually, apparently some folks have used it with success by only leaving it down for a couple of hours, then vacuuming up. However, I had *such* a persistent flea infestation, and had already spent so much money on products to treat the problem, that I left it down for two weeks. I also dusted the cats with it, working it into their fur and being careful that they didn’t inhale any. I also dusted the upholstered chair & ottoman with it, and didn’t vacuum until 2 weeks later.
The flea life cycle is a bit involved, but basically, those 9 fleas you saw on your cat (if that’s every single flea) indicate a fairly serious infestation. Adult fleas are typically only 5% of the population; flea population is typically made up of 50% eggs, 30% larvae, 15% pupae and 5% adults.
I leave the stuff down thick enough and long enough to break the whole lifecycle, not just the adults that already exist. I want to catch the larvae and pupae as they come out. And eggs that hatch into larvae. Eggs hatch anywhere from two days to two weeks after being laid. They’re typically found indoors in floor cracks & crevices, along baseboards, under rug edges and in furniture or beds. As well as on the pets!
(So I leave the stuff down for 2 weeks, figuring that’s doing a better job of breaking the lifecycle.)
Basically, if you have items that are upholstered and/or not washable (rugs, curtains, mattress, cat bed, decorative pillows, etc.) it needs to be treated.
You DON’T want to breathe this stuff. I wore a respirator when I was applying it, which freaked out the kittens a little bit. But my cotton gloves took up plenty of DE when I dipped them in the bag, and then rubbed it into the cat fur.
You DON’T want swimming pool grade diatomaceous earth. (The particles are too large to work effectively.) You want food grade diatomaceous earth. If you find horticultural grade, be sure to check the labeling to make sure there’s NO PESTICIDE in it. There shouldn’t be anything in the list of contents except diatomaceous earth.
I purchased my from a local lawn care store (Suburban Lawn & Garden), but you can purchase it online. I bought a 4 lb bag, and it was plenty. I found it at:
http://www.agecology.com/products/showprod.php?ProdID=252
http://www.dirtworks.net/Diatomaceous-Earth.html (fossil shell flour)
http://www.extremelygreen.com/Product.cfm?Name=Diatomaceous%20Earth%20
Good luck! It really does work; within 3 days my cats had stopped scratching, and they’ve not had fleas since.
(Maybe the large cracks between my floorboards has something to do with it.)