Find the total length of a youtube playlist with javascript console.

Youtube Playlist Duration

// remove all comments lines

var countUdTime = 0;
var ytt = "desktop";

// for Desktop / laptop / youtube.com
// ===================================

// on YouTube Videos Page - https://www.youtube.com/channel/UCapQ8VzljfmUW5OM-ynsyCQ/videos
var ytp = document.querySelectorAll("ytd-grid-video-renderer");

if (ytp.length < 1) {
    // on Playlist Page - https://www.youtube.com/playlist?list=PLv7XeeXKl8EKbJ-McS0ZUi1iP9ja12yaa
    ytp = document.querySelectorAll("ytd-playlist-video-list-renderer > #contents > ytd-playlist-video-renderer");
}
if (ytp.length < 1) {
    // on Playlist Watch Page - https://www.youtube.com/watch?v=MlBXz9gv08E&list=PLv7XeeXKl8EKbJ-McS0ZUi1iP9ja12yaa&index=1
    ytp = document.querySelectorAll("ytd-playlist-panel-video-renderer");
}

// for Mobile / m.youtube.com
// ==========================

if (ytp.length < 1) {
    ytt = "mobile";
  
    // on YouTube Videos Page - https://m.youtube.com/c/UCapQ8VzljfmUW5OM-ynsyCQ/videos/
    ytp = document.querySelectorAll("ytm-compact-video-renderer");    

    if (ytp.length < 1) {
        // on Playlist Page 
        ytp = document.querySelectorAll("ytm-playlist-video-list-renderer > ytm-playlist-video-renderer");
    }
    if (ytp.length < 1) {
        // on Playlist Watch Page
        ytp = document.querySelectorAll("ytm-playlist-panel-video-renderer");
    }
}

var time = 0;
for (var i = 0; i < ytp.length; i++) {
    try {
        if( ytt == "mobile"){
          var a = ytp[i].getElementsByTagName("ytm-thumbnail-overlay-time-status-renderer")[0].innerText;
        }else{
          var a = ytp[i].getElementsByTagName("ytd-thumbnail-overlay-time-status-renderer")[0].innerText;
        }
        
        var tx = a.split(":");
        if (tx.length < 3) {
            time = time + Number(tx[0]) * 60 + Number(tx[1]);
        } else if ((tx.length = 3)) {
            time = time + Number(tx[0]) * 60 * 60 + Number(tx[1]) * 60 + Number(tx[2]);
        }
    } catch (e) {
        countUdTime = countUdTime + 1;
    }
}
var ytpT = convertS(time);
var show = "YouTube Playlist \n --------------- \n Total Videos : " + ytp.length + "\n Total Duration : " + ytpT + "\n Avg. Duration  : " + convertS(time / ytp.length);

alert(show);
alert("Error to Get Time : " + countUdTime + " videos");
function convertS(sec) {
    var hrs = Math.floor(sec / 3600);
    var min = Math.floor((sec - hrs * 3600) / 60);
    var seconds = sec - hrs * 3600 - min * 60;
    seconds = Math.round(seconds * 100) / 100;
    var result = (hrs < 10 ? "0" + hrs : hrs) + " Hours ";
    result += (min < 10 ? "0" + min : min) + " Min ";
    result += (seconds < 10 ? "0" + seconds : seconds) + " Sec ";
    return result;
}

alternative : https://ytplaylist-len.herokuapp.com/