There is variation on how often Applets run, depending on the type of Applet and the service APIs involved. Some will run within seconds, while others may take up to an hour.
On average, most Applets run within 15 minutes of being triggered.
If you click the "Check Applet" button when you are viewing an Applet that is turned on, it will force the Applet to run. If the Applet does not identify new data to trigger from, it will not run. Try clicking "Check Applet" at a later time.