//paste and execute in console on *channel link*/videos
//change 'to' to the desired number of video length values

var to = 20;    //number of videos
var list = [];  //length array

for(i = 1;i<=to;i++){
    list[i-1] = $("#items > ytd-grid-video-renderer:nth-child("+i+") > #dismissable > ytd-thumbnail > #thumbnail > #overlays > ytd-thumbnail-overlay-time-status-renderer > span.ytd-thumbnail-overlay-time-status-renderer").html();   //get video length from span
    list[i-1] = list[i-1].replace(/\s/g, '');   //get rid of whitespace

    var p = list[i-1].split(':'),
        s = 0, m = 1;

    while (p.length > 0) {
        s += m * parseInt(p.pop(), 10);
        m *= 60;
    }

    list[i-1] = s;  //convert to seconds

}

var a = document.createElement('a');
var file = new Blob([ list.toString() ], { type: 'text/plain' });

a.href = URL.createObjectURL(file);
a.download = 'source';
a.click();  //download to file "source.txt"