When do AdMob test ads start appearing?

I just followed the guide for setting up ads for my game in Unity (https://developers.google.com/admob/unity/start). I've set up my app and a banner ad unit in AdMob. I have the script set up in Unity to use the test banner ID. I verified that the request is being created and sent when I run the app in the Unity editor. However, when I build and run the app on my android device, no ads appear. The error message says "no fill," which I guess means there was no add to fill the space. But why would that be the case, when I'm requesting a test banner?

The AdMob help section says it can take "some time" for live ads to begin appearing, but that's not very helpful. I don't know whether "some time" means ten minutes, ten hours, or ten days. In the meantime, I can't tell whether I set everything up correctly and I just need to wait patiently, or if no ads are appearing because I did something wrong. How long should I wait before assuming something's not working correctly? Is there any other way I can tell that it's working? Is there any way I can speed up the process? (Like, does it populate with ads more quickly if it receives more ad requests?)

Thanks for the help!

 

I have the same issue since couple days - ads loading properly, but real ad units. Test add units returns "no fill" error. A week ago everything was working properly.

 

The problem I am facing is a little bit different, for me banner test ads show up but not interstitial neither rewarded videos. That's completely weird. Do you have any idea why this is happening