How long does it take flea medicine to work?

I keep combing my cat, and I always find one flea on her after I apply medication. I have carpet throughout my home, and I’m unsure whether I should use a flea fogger or buy a carpet powder and vacuum the carpet. Should I consider hiring an exterminator? What’s the best course of action? Any advice would be appreciated.

Topical flea and tick treatments, like Revolution Plus, kill fleas and ticks within 12 hours. Within 24 hours, at least 98% of fleas are dead. Oral flea medicines like Capstar start killing fleas in 30 minutes and keep doing so quickly for 24 hours.

It is a chemical or a medicine that does not effect the animal, but it deadly (or a nuisance) to insects - so the dog absorbs it thru their skin and it flows thru their blood stream around to all the cells and any bug that jumps on them either bites them and dies OR the scent of it is detectable by the insect and they just wont bite the animal for fear of that chemical/medicine.