That's probably a very grey area heading towards subliminal advertising (at least, some lawyer somewhere could claim so). I'm not sure on the legal status of that sort of thing.

I'm noticing very short ads on YouTube (about 5sec) which are cheaper and more effective (ime) than the longer ones, which are usually skipped before they communicate the message and cause (more) annoyance.

I've seen a few like that, and they're good. I've seen some which cram a lot into 5 seconds, and then tell you you can skip the rest.

But I've also seen weirdly long ads - 10 minute ad on a 7 minute video, 30 minute ad on an hour long video.

I'm also noticing some ads have a loud noise at the beginning of a 30 second skippable ad.

https://www.youtube.com/watch?v=Dxg7fse74MY

https://www.youtube.com/watch?v=1crwU9aUNHE

It's really annoying.

This is the kind of thing that drives people to ad-blocking.

We're in the realm of speculation here, not a lawyer etc. 5 seconds is a fairly decent amount of time if you count it out. Plenty of time to register what you're seeing.

1 second (and below) it might be possible to argue it's too short to register the advert consciously, making it a subliminal advert. Again I don't know if this is true and I'm completely making this up. It'd be interesting to find out the answer.

A second is enough time to consciously register an ad. To prove it, they could run a bit where someone shouts "50 percent?!" with a picture of an iPad and "Tap! 50% off". I'll bet people start tapping by the second or third impression (even though it's obviously a scam of some kind).