This is just a little snippet I discovered in the WordPress codex that allows your to display the time since your post was published. I wrote it into a little function that you can add to your functions.php file:

/**
 * Display time since post was published
 *
 * @uses	human_time_diff()  Return time difference in easy to read format
 * @uses	get_the_time()  Get the time the post was published
 * @uses	current_time()  Get the current time
 *
 * @return	string  Timestamp since post was published
 *
 * @author c.bavota
 */
function get_time_since_posted() {

	$time_since_posted = human_time_diff( get_the_time( 'U' ), current_time( 'timestamp' ) ) . ' ago';

	return $time_since_posted;

}

Now you just need to include get_time_since_posted() in the WordPress loop to display the time since your post was published.

Read more about human_time_diff() at http://codex.wordpress.org/Function_Reference/human_time_diff.