cache_form expiration is configurable in Drupal 7

In Drupal 7, the expiration for entries in the {cache_form} database table was previously hardcoded to be 21600 second (6 hours). This is now configurable in settings.php:

/**
 * Expiration of cache_form entries:
 *
 * Drupal's Form API stores details of forms in cache_form and these entries are
 * kept for at least 6 hours by default. Expired entries are cleared by cron.
 * Busy sites can encounter problems with the cache_form table becoming very
 * large. It's possible to mitigate this by setting a shorter expiration for
 * cached forms. In some cases it may be desirable to set a longer cache
 * expiration, for example to prolong cache_form entries for Ajax forms in
 * cached HTML.
 *
 * @see form_set_cache()
 * @see system_cron()
 * @see ajax_get_form()
 */
# $conf['form_cache_expiration'] = 21600;