<video id="myVideo" width="320" height="176" controls ondurationchange="myFunction(this)">
  <source src="https://www.sample-videos.com/video321/mp4/240/big_buck_bunny_240p_5mb.mp4" type="video/mp4">
  <source src="https://www.sample-videos.com/video321/mp4/240/big_buck_bunny_240p_5mb.mp4" type="video/ogg">
  Your browser does not support HTML5 video.
</video>

<script>
function myFunction(x) {
  alert("The duration of this video is : " + x.duration + " seconds");
}
</script>