When your users follow your calendar, an unique URL to your calendar is stored in the users calendar client (Google Calendar, Outlook, Exchange, Apple Calendar etc).

That unique URL is requested by the calendar client frequently. The frequency of that request / sync depends on the calendar client.

Our experience in sync frequency is:

• Apple updates on app / program startup and every 1-3 hours
• Google updates every 12 hours
• Outlook updates on app / program startup and every 1-3 hours
• Outlook.com updates every 3 hours
• Yahoo updates every 8-12 hours