Quick and dirty cache expiration testing. I placed in functions.php just for a quick place to put it. Refresh the page, and it should change every 10 seconds.

quick-and-dirty-cache-expire-test.php

<?php

if (isset($_GET['matt'])) {
    $cur_time = time();
	$cache = wp_cache_get('matt-time', 'matt');
	if ($cache !== false) {
		echo '<pre>Found'.print_r($cache, 1).'</pre>';
	}
	else {
		wp_cache_set('matt-time', $cur_time, 'matt', 10);
		$cache = wp_cache_get('matt-time', 'matt');
		echo '<pre>Setting'.print_r($cache, 1).'</pre>';

	}
	die();
}