JSFiddle - React, Tailwind, and code Playground

HTML

<div class="block"></div>

CSS

.block {
  width: 200px;
  height: 200px;
  background-color: blue;
}

JavaScript

const block = document.querySelector('.block');
let time = 0;

block.addEventListener('mouseover', () => {
	time = performance.now()
});

block.addEventListener('mouseout', () => {
	alert(performance.now() - time)
});