Delete transients older than 7 days.

functions.php

<?php
class PurgeTransients {

	var $expire = '7 days';

	function __construct(){
		add_action( 'admin_init', array( &$this, 'init' ) );
	}
	function init(){
		$this->purge( $this->expire );
	}
	function purge( $older_than ) {
		global $wpdb;

		$older_than_time = strtotime('-' . $older_than);
		if ( $older_than_time > time() || $older_than_time < 1 ) {
			return false;
		}
		$transients = $wpdb->get_col(
			$wpdb->prepare( "
					SELECT REPLACE(option_name, '_transient_timeout_', '') AS transient_name
					FROM {$wpdb->options}
					WHERE option_name LIKE '\_transient\_timeout\__%%'
						AND option_value < %s
			", $older_than_time )
		);

		if( ! is_array( $transients ) )
			return;
		$option_names = array();
			foreach( $transients as $transient ) {
				$option_names[] = '_transient_' . $transient;
				$option_names[] = '_transient_timeout_' . $transient;
			}
		if ( ! empty( $option_names ) ) {
			foreach( $option_names as $option )
				$result = $wpdb->query( "DELETE FROM {$wpdb->options} WHERE option_name = '{$option}'" );
		}
	}
}
new PurgeTransients;