Run this in console of youtube history page (after expanding out history) to get count of video time

youtubehistory.js

var data=document.getElementsByClassName('video-time');
var re = /^([2][0-3]|[0-1]?[0-9])([.:][0-5]?[0-9])?$/;
var total=0;
for(var i=0;i<data.length;i++){
  var j=data.item(i).innerText;
  var match=re.exec(j);
  if (match != null) {
    total += parseInt(match[1])
  }
}