diff --git a/core/lib/Drupal/Core/Form/FormCache.php b/core/lib/Drupal/Core/Form/FormCache.php
index 7064e10..9c931fd 100644
--- a/core/lib/Drupal/Core/Form/FormCache.php
+++ b/core/lib/Drupal/Core/Form/FormCache.php
@@ -8,6 +8,7 @@
 use Drupal\Core\KeyValueStore\KeyValueExpirableFactoryInterface;
 use Drupal\Core\PageCache\RequestPolicyInterface;
 use Drupal\Core\Session\AccountInterface;
+use Drupal\Core\Site\Settings;
 use Psr\Log\LoggerInterface;
 use Symfony\Component\HttpFoundation\RequestStack;
 
@@ -170,8 +171,8 @@ protected function loadCachedFormState($form_build_id, FormStateInterface $form_
    * {@inheritdoc}
    */
   public function setCache($form_build_id, $form, FormStateInterface $form_state) {
-    // 6 hours cache life time for forms should be plenty.
-    $expire = 21600;
+    // Cache forms for 6 hours by default.
+    $expire = Settings::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
diff --git a/sites/default/default.settings.php b/sites/default/default.settings.php
index 25d498e..8111073 100644
--- a/sites/default/default.settings.php
+++ b/sites/default/default.settings.php
@@ -440,6 +440,20 @@
  */
 # $settings['cache_ttl_4xx'] = 3600;
 
+/**
+ * Expiration of cache_form entries.
+ *
+ * Drupal's Form API stores details of forms in a cache 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 \Drupal\Core\Form\FormCache::setCache()
+ */
+# $conf['form_cache_expiration'] = 21600;
 
 /**
  * Class Loader.