diff --git a/calendar/inc/class.calendar_so.inc.php b/calendar/inc/class.calendar_so.inc.php
index e6d37ba01f..bd5f1ea7f7 100644
--- a/calendar/inc/class.calendar_so.inc.php
+++ b/calendar/inc/class.calendar_so.inc.php
@@ -498,7 +498,7 @@ class calendar_so
 	/**
 	 * Maximum time a ctag get cached, as ActiveSync ping requests can run for a long time
 	 */
-	const MAX_CTAG_CACHE_TIME = 300;
+	const MAX_CTAG_CACHE_TIME = 29;
 
 	/**
 	 * Get maximum modification time of events for given participants and optional owned by them