This field "endAt" is not the same as the "duration" of the video, but where the playlist owner wants it to stop playing. The video duration is located in the video resource and not in the playlistItem. And it requieres another lookup request.
You can find the definitions of the fields in the YouTube documentation. See for those resource definitions: https://developers.google.com/youtube/v3/docs/playlistItems#resource and https://developers.google.com/youtube/v3/docs/videos#resource