Fungicide question

Hey all, I had an exterminator come and spray my house twice for BB. It's been since early October, they sprayed two weeks ago. We're still getting bit. They use a type of fungicide that leaves a residue for the BB to walk in and spread to others and kills them. The question: if anyone's used a similar method- how long did it take for the bugs to totally die out? This just feels impossible and I'm looking for some hope that it just takes a while.