YoutubePlaylistTotalTime.js

// ==UserScript==
// @name         Youtube Playlist Total Time
// @namespace    https://www.youtube.com/
// @version      0.3
// @description  Shows cumulative time of videos in a Youtube playlist
// @author       Paolo Simone
// @match        https://www.youtube.com/playlist*
// @grant        none
// @run-at       document-start
// ==/UserScript==

// credits: https://gist.github.com/liaohuqiu/4ee77b9b03afcdecc80252252378d367
(function() {
  function toSeconds(hms) {
    var a = hms.split(':');
    while (a.length < 3) {
      a.unshift(0);
    }
    var seconds = (+a[0]) * 60 * 60 + (+a[1]) * 60 + (+a[2]);
    return seconds;
  }

  function format(totalSec) {
    let days = parseInt(totalSec / 3600 / 24);
    let hours = parseInt(totalSec / 3600) % 24;
    let minutes = parseInt(totalSec / 60) % 60;
    let seconds = totalSec % 60;
    return (days > 0 ? (days + "d ") : "") +
        (hours > 0 ? (hours + "h ") : "") +
        (minutes > 0 ? (minutes + "m ") : "") +
        seconds + "s ";
  }

  function computeTotalTime(items) {
    let totalSeconds = 0;
    for (let i = 0; i < items.length; i++) {
      totalSeconds += toSeconds(items[i].innerText);
    }
    return format(totalSeconds);
  }

  function showTotalTime(lastLoaded) {
    let timeLabels = document.querySelectorAll('span.ytd-thumbnail-overlay-time-status-renderer');
    let fullyLoaded = timeLabels.length > 0 && timeLabels.length === lastLoaded;

    if (!fullyLoaded) {
      setTimeout(() => showTotalTime(timeLabels.length), 200);
      return;
    }

    let timeSpan = document.createElement("span");
    timeSpan.innerText = ` • ${computeTotalTime(timeLabels)}`;

    let stats = document.querySelectorAll('.metadata-stats > yt-formatted-string')[0];
    stats.appendChild(timeSpan);
  }

  window.addEventListener('load', () => showTotalTime(0), false);
})();