diff --git a/includes/form.inc b/includes/form.inc
index e749239..3b2032e 100644
--- a/includes/form.inc
+++ b/includes/form.inc
@@ -555,8 +555,10 @@ function form_get_cache($form_build_id, &$form_state) {
  * Stores a form in the cache.
  */
 function form_set_cache($form_build_id, $form, $form_state) {
-  // 6 hours cache life time for forms should be plenty.
-  $expire = 21600;
+  // The default cache_form expiration is 6 hours. On busy sites, the cache_form
+  // table can become very large. A shorter cache lifetime can help to keep the
+  // table's size under control.
+  $expire = variable_get('form_cache_expiration', 21600);
 
   // Ensure that the form build_id embedded in the form structure is the same as
   // the one passed in as a parameter. This is an additional safety measure to
diff --git a/sites/default/default.settings.php b/sites/default/default.settings.php
index b044be3..a152b28 100644
--- a/sites/default/default.settings.php
+++ b/sites/default/default.settings.php
@@ -479,6 +479,23 @@ ini_set('session.cookie_lifetime', 2000000);
 # $conf['block_cache_bypass_node_grants'] = TRUE;
 
 /**
+ * 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;
+
+/**
  * String overrides:
  *
  * To override specific strings on your site with or without enabling the Locale