It's really dependent on video. Some videos can be 20 minutes long and you have a skippable ad in the beginning only. Other videos are only ~6 minutes long and play an unskippable ad every 45 seconds.
It probably depends on monetization settings with the channel holder, but it's really annoying. The former kind, I don't mind, but the latter is almost as bad as cable TV.
Maybe it's a little different depending on your creator status, who you're partnered with, etc - but from my experience of monetising videos, there is very little control over the ads, for example, here's what I see when I choose to monetise a video.
It probably depends on monetization settings with the channel holder, but it's really annoying. The former kind, I don't mind, but the latter is almost as bad as cable TV.