<video width="320" height="240" id = "myVideo" controls>
  <source src="someVideo.mp4" type="video/mp4">
Your browser does not support the video tag.
</video>

<script>
var e = document.getElementById("myVideo");
var cancelAlert = false;
var run = function() {
if (e.readyState === 4 && !cancelAlert) {//readyState 4 means it is loaded fully
cancelAlert = true;//This is so that it only alerts once. No spam pls.
alert(e.duration);
}
requestAnimationFrame(run);//This is so that it runs every frame.
};
run();//run the function
</script>