I’ve seen big results within 3 or 4 days of beginning treatment, especially with really bad infestations, but you MUST stick with the two weeks to really see good results.
And when I say to be generous with the environmental treatment, I mean it should really look like a flour bomb exploded in your house or apartment – especially if you have carpeting or upholstered furniture! If you sleep on a mattress or futon, you’ll need to treat it before putting a cover or sheet on. Anywhere the pet habitually sleeps should either be coated thickly or the bedding should be put through the wash on hot (and dried on high). Everything should feel a bit dusty or higher-friction than previously.
You can vacuum, sweep, whatever in a couple of weeks. But in the meantime, your pet should get a bit more DE every couple of days (including the belly, sides, under the jaw, tail, armpits, etc.).
It’s not an instantaneous way to kill fleas, larvae, or their eggs. But it’s very effective, non-toxic, and impossible for fleas to develop resistance to.