yt-watch-later-duration.js

var elements = Array.from(document.querySelectorAll(".ytd-playlist-video-list-renderer span.ytd-thumbnail-overlay-time-status-renderer"))
var time = elements.map(v => {
  const [s,m,h] = v.innerText.split(':').reverse().map(x => parseInt(x,10));
  return s + (m || 0) * 60 + (h || 0) * 60 * 60;
 })
var total = time.reduce((acc, v) => acc + v);
var h = Math.floor(total / (60 * 60));
var m = Math.floor((total - (h * 60 * 60)) / 60);
var s = total%60;

console.log(`${h.toString().padStart(2, '0')}:${m.toString().padStart(2, '0')}:${s.toString().padStart(2, '0')}`);