They say that it takes three days to work. After that, generally, they start dropping off or you will find them dead and such. I know that when I dose my critters they like to flee (hehe. get it? flee, flea. ) the immediately dosed area for a while then they aren't around at all, generally, after that period of a few days.

If that doesn't work then maybe try some home remedies for fleas? Dosing them more than once could be harmful, I imagine.