How to measure a time spent on a page?

The accepted answer is good, but (as an alternative) I’ve put some work into a small JavaScript library that times how long a user is on a web page. It has the added benefit of more accurately (not perfectly, though) tracking how long a user is actually interacting with the page. It ignore times that a user switches to different tabs, goes idle, minimizes the browser, etc. The Google Analytics method suggested in the accepted answer has the shortcoming (as I understand it) that it only checks when a new request is handled by your domain. It compares the previous request time against the new request time, and calls that the ‘time spent on your web page’. It doesn’t actually know if someone is viewing your page, has minimized the browser, has switched tabs to 3 different web pages since last loading your page, etc.

Edit: I have updated the example to include the current API usage.

Edit 2: Updating domain where project is hosted

https://github.com/jasonzissman/TimeMe.js/

An example of its usage:

Include in your page:

<!-- Download library from https://github.com/jasonzissman/TimeMe.js/ -->
<script src="https://stackoverflow.com/questions/4667068/timeme.js"></script>
<script type="text/javascript">
TimeMe.initialize({
    currentPageName: "home-page", // page name
    idleTimeoutInSeconds: 15 // time before user considered idle
});
</script>

If you want to report the times yourself to your backend:

xmlhttp=new XMLHttpRequest();
xmlhttp.open("POST","ENTER_URL_HERE",true);
xmlhttp.setRequestHeader("Content-type", "application/x-www-form-urlencoded");
var timeSpentOnPage = TimeMe.getTimeOnCurrentPageInSeconds();
xmlhttp.send(timeSpentOnPage);

TimeMe.js also supports sending timing data via websockets, so you don’t have to try to force a full http request into the document.onbeforeunload event.