The speed in which you see a change will depend on the pest or rodent in question. For the more resilient creatures like rodents and cockroaches, it can take a few days and sometimes even weeks. Treatments have improved over recent years and the timespan continues to decrease.
For other creatures like perhaps mice or ants, they may be gone the same day as the treatment. It will all depend on the type of pest in question and how they must be handled.