1. Electronic pest control never has instant effect like when you e.g. turn on the light!
Electronic rodent control can take anything from a few days up to, in extreme cases several months.The time factor mainly depends on;
a) to what extent the rodents are exposed to the sound waves,
b) how tough the fight with other rodents will get concerning alternative survival territory, as well as
c) in what way the predominant weather type will allow the rodents to move out (frost in combination with no snow might slow the eviction process considerably!)
Moral: Only consider pest control appliances that comes with ample Money-Back-Guarantee. In practice this means your vendor should grant you a trial period of at least 6 months!