RadNotification notification =
notification.LoadContentOn = Telerik.Web.UI.NotificationLoad.TimeInterval;
notification.UpdateInterval = 20000;
sender, RadNotificationEventArgs e)
( <some condition>)
You can use the OnClientUpdated event to show the notification if meaningful information is returned, as shown here: http://demos.telerik.com/aspnet-ajax/notification/examples/updateinterval/defaultcs.aspx.
The Show() call cannot work during the callback because it has to register a script that will show the notificaiton and this is impossible during a callback.
It is also important to recreate the notifrication each time the page loads (e..g, in the Page_Init or Page_Load event).
Check out the Telerik Platform - the only platform that combines a rich set of UI tools with powerful cloud services to develop web, hybrid and native mobile apps.