Calcula o tempo total de uma playlist no Youtube, com condicional para vídeos apenas não assistidos. É só colar no console na página da playlist. Vi a ideia de um colega aqui do Git Hub e aprimorei.

playlist_time_with_conditional.js

(function () {

    var onlyUnassistedVideos = true;

    var timeSeconds = 0;

    var timestampDivList = document.querySelectorAll("#pl-load-more-destination");

    for (var i of timestampDivList[0].children) {

        var percentageGreaterOrAlmostHundred = false;

        if (onlyUnassistedVideos) {

            for (var l of i.children) {
                
                if(l.className == "pl-video-thumbnail"){
    
                    // resume-playback-progress-bar: class who only have percentage relative to quantity of watched video
                    for(var m of l.children[0].children) {
                        if(m.className == "resume-playback-progress-bar"){ 
                            var percent = m.style.cssText.replace(/\D/g, '');
    
                            console.log('procentagem assitida: ', percent);
    
                            if(percent >= 95) {
                                percentageGreaterOrAlmostHundred = true;
                            }
    
                        }
                    }
                }
                
            }
        }

        for (var j of i.children) {
            
            // pl-video-time: class relative to time video
            if(j.className == "pl-video-time"){
                
                var timeParts = j.innerText.split(":");
                
                if(onlyUnassistedVideos && !percentageGreaterOrAlmostHundred) {

                    timeSeconds += (timeParts[0] * 60) + parseInt(timeParts[1]);
                } else if(!onlyUnassistedVideos) {

                    timeSeconds += (timeParts[0] * 60) + parseInt(timeParts[1]);
                }
    
            }
            
        }

    }
        
    var hours = (timeSeconds / 60) / 60;
    var minutes = (timeSeconds / 60) % 60;
    var seconds = (timeSeconds % 60); 
    
    var result = parseInt(hours) + ":" + parseInt(minutes) + ":" + parseInt(seconds);
    
    alert(result);

})();