When I used the Borax, I left it down about 24 hours. But it sounds like your problem is outside and not inside. I am not sure what Borax will do to lawn or landscaping, but you could try a ring of it around your home. I'd suggest about a two foot wide ring. I know that's a lot of Borax, but it's the safest, quickest and most reliable product I have found.
You may also try calling a local exterminator (you may have to call more than one) and ask if they spray for fleas and what kind of product do they use. There are some natural products available to them that will dry up fleas, but are harmless to our pets.