I tested out Peacock's ad-supported free tier and its ad-supported $5-a-month Premium tier. Peacock promises that you'll see five minutes or less of ads per hour across both ad-supported tiers.
My experience varied depending on the show and device. While watching The Hitman's Bodyguard on a Roku TV, there were six ads sprinkled throughout the film, ranging from 20 to 60 seconds each. Peacocktv.com/tv even marks midroll ad breaks so you know when to expect them. But when it played on the iPhone app, there was a notice that we would watch 135 seconds of ads at the beginning, and none for the rest. That option would be great to have on the Apple TV too to get the ads out of the way, but unfortunately it's not (yet).
After scrolling around and watching a bunch of ads, when I went back to start The Hitman's Bodyguard again, there were no ads at all, because I had already seen 5 minutes' worth in the previous hour. It does seem like if you pop in and out of a movie or show, the ad count may reset. When I streamed the movie Nope, there was only a 2.5-minute set of ads before the movie, with no commercials during the film. And The Godfather's three-hour runtime didn't have any ad interruptions.
On episodes of Saturday Night Live, there were seven to nine ads sprinkled throughout the episode on both mobile and TV. Modern Family had three to four ad breaks within one 23-minute episode. This is about the same ad experience as watching on Hulu's $8-a-month ad-supported plan, or on regular live TV.
It's also worth mentioning that some subscribers to the most-expensive, ad-free, paid version will still see ads on "a small amount of programming, Peacock.com/tv channels, live events and a few TV shows and movies," according to Peacock.