See video time of a youtube iframe?

On my website, I have a iframe that is entered in the database for submission and I would like to see the length of the iframe video. However, I tried to use Youtube API but I did not succeed. The iframe gets a id after page loading due to organization issues (I know it's not practical but it's the way I got it).

Here's the code.

  // 2. This code loads the IFrame Player API code asynchronously.
  var tag = document.createElement('script');

  tag.src = "https://www.youtube.com/iframe_api";
  var firstScriptTag = document.getElementsByTagName('script')[0];
  firstScriptTag.parentNode.insertBefore(tag, firstScriptTag);

  // 3. This function creates an <iframe> (and YouTube player)
  //    after the API code downloads.
  var player;
  function onYouTubePlayerAPIReady() {
       player = new YT.Player('video_player');
       time_left = player.getDuration();
       alert(time_left);
    }

  // 4. The API will call this function when the video player is ready.
  function onPlayerReady(event) {
    event.target.playVideo();
  }

  // 5. The API calls this function when the player's state changes.
  //    The function indicates that when playing a video (state=1),
  //    the player should play for six seconds and then stop.
  var done = false;
  function onPlayerStateChange(event) {
    if (event.data == YT.PlayerState.PLAYING && !done) {
      setTimeout(stopVideo, 6000);
      done = true;
    }
  }
  function stopVideo() {
    player.stopVideo();
  }

video_player is the iframe that I choose to see the duration. Here I have the code where I change the id of the iframe.

if($("iframe").length > 0){
    $("iframe").attr("id", "video_player");
    $("iframe").attr("width", "100%");
    $("iframe").attr("height", "100%");
}