That’s when I copy the URL, close the window, and download the video without the ad.
Downie for MacOS or yt-dlp at the command line. When those stop working, and if there’s no alternative, I will stop watching YT videos. I will not be harassed by ads. There’s already more content than I have time to consume.
I like having a YouTube account because it keeps track of which videos I've watched across devices. I'm not thrilled at Google knowing that information, of course, but I hate that every single privacy-conscious YouTube alternative fails to provide for that use-case.