y-playlist-duration.js

{
  const formatS = (s = 0) =>
    `${Math.floor(s / 3600)}:${`${Math.floor(s / 60) % 60}`.padStart(2, 0)}:${`${s % 60}`.padStart(2, 0)}`;

  const getPlaylistDuration = () => {
    const videos = [
      ...document
        .querySelector('ytd-playlist-video-list-renderer')
        .parentElement.querySelectorAll('span.ytd-thumbnail-overlay-time-status-renderer'),
    ];
    const videosDuration = videos.map((video) => {
      const [s = 0, m = 0, h = 0] = video.textContent.split(':').map(Number).reverse();
      return 3600 * h + 60 * m + s;
    });

    return formatS(videosDuration.reduce((a, b) => a + b, 0));
  };

  console.clear();
  console.log(`Playlist duration: ${getPlaylistDuration()}`);
}