It sounded to me like 20 minutes of adverts in total, not one trailer that length.
It’s not really all that clear (imagine that, a news reporter not being clear!). But the way that the article said “A Chinese woman is suing a cinema and a film’s distributors for wasting her time by showing 20 minutes of adverts. . .” and then “. . .Huayi Brothers Media Corporation should have told her how long the advertisements for the film Aftershock lasted”, I took it to mean that there were 20 minutes of advertisements for that one movie.
Hmmmm. . .okay. After looking at a few other articles, it seems that it wasn’t even a trailer for a movie. It was 20 minutes of just the sort of ordinary commercial advertisements that you see on television, for such products as drinks or cars or whatever. And then came the 135 minutes of the movie “Aftershock”.
And apparently the woman watched the movie twice. Which gave her a total of 40 minutes of watching car commercials and Coca-Cola ads.