Note that this is talking about ads when you watch via their TV app, not when you watch on your computer in an ordinary browser.
The ad experience in the browser is interesting. I've watched several videos today with no ad blocking and have only seen a couple of ads which were skippable after 5 seconds.
For the last year I've doing this on YouTube.
• I start the day with ad blocking turned off.
• As long as all the ads I get are skippable after 5 seconds, and any ads that are not pre-roll ads are not too close to the previous ad, I just skip and my annoyance level stays low. I get annoyed if the ads during the content are not well spread out.
• If they put in a non-skippable ad but it is close to 5 seconds (6 seconds and 15 seconds seem to be the two shortest lengths for non-skippable ads) and it is not followed immediately by another ad I treat it like a 5 second skippable ad.
• If a 6 second non-skippable ad is followed immediately by a skippable after 5 seconds ad, my annoyance level rises some. If they only do this a couple times in a day I'll probably let it slide.
• Any 15 second or more non-skippable ad raises my annoyance level a lot.
• If my annoyance level gets too high, by either (1) a 15+ second non-skippable ad or (2) too many of those things that are annoying but not as annoying then ad blocking gets turned on any open YouTube tabs get refreshed so it will apply to them.
• Any ad they show me that is actually relevant and useful is not counted when evaluating annoyance level. This only happens rarely.
I haven't kept stats but it definitely feels like over the first couple of months of doing this they changed the ad lengths, intervals, and how often they show me non-skippable ads in a way that avoids annoying me enough to turn on ad blocking. I now sometimes go over a week without turning on the ad blocker.