How to track user time on site

I'm looking to track users average time on a website (in the same way that Google analytics does) for internal administration.

What's the easiest way to do this?

Solution

You can get the time in next ways:

Once user visit your site, save current time at cookie as "visited", and at next visit you can grab it, if it was set.
And more expensive method: when the page loads, start js timer, and on page unload send to server time which user sent and save it to db.
And if window.unload does not work at Opera, you can send time to server every 5 seconds, and stores it to DB.

If you need, I can write an example script.

UPDATE:

<!DOCTYPE html>
<html>
    <head>
        <title>Collect time</title>
        <script type="text/javascript" src="jquery-1.4.2.min.js"></script>
        <script type="text/javascript">
        $(function()
        {
            var start = null;
            $(window).load(function(event) {
                start = event.timeStamp;
            });
            $(window).unload(function(event) {
                var time = event.timeStamp - start;
                $.post('/collect-user-time/ajax-backend.php', {time: time});
            })
        });
        </script>
    </head>
    <body>

    </body>
</html>

And backend script:

<?php 
$time = intval($_POST['time']);
if (!file_exists('data.txt')) {
    file_put_contents('data.txt', $time . "\n");
} else {
    file_put_contents('data.txt', $time . "\n", FILE_APPEND);
}

But as I said it wouldn`t work at Opera browser