The video tag has a "duration" property, which will tell you the length of a video.
https://www.w3schools.com/tags/av_prop_duration.asp
<html>
<body>
<button onclick="myFunction()" type="button">Get video length</button><br>
<video id="myVideo" width="320" height="176" controls>
<source src="https://firebasestorage.googleapis.com/v0/b/qriositynet-dev.appspot.com/o/chat%2FMgttfKqKIDhQ6bgtgy6V%2Fvideos%2F1663229371400watermelon-bunny.mp4?alt=media&token=722bb260-c65b-46fe-8805-4a5a742f282d" type="video/mp4">
</video>
<script>
function myFunction() {
let vid = document.getElementById("myVideo");
alert(vid.duration);
}
</script>
</body>
</html>