Welcome to Android Central! I don't use Kijiji, but if it's coded well, then it should trigger a notification as soon as the appropriate criteria is met. However, delays could be caused by the phone itself, if the firmware designed to aggressively kill apps in the background (in a misguided attempt to save battery). Samsung and Huawei phones are fairly notorious for this kind of thing:
https://dontkillmyapp.com/. One of the things you'd have to do is to make sure Battery Optimization is turned off for the app in the system settings.