How often do calendars sync?
It depends on the type of source calendar.
Google & Outlook source calendars — real time
Sync connections where the source calendar is a Google or Outlook calendar are “real-time” and work based on push notifications (rather than syncing at periodic intervals). This means that changes on a source calendar typically propagate to the destination calendar within a minute — but can take longer depending on factors such as network conditions and the number of events on a calendar.
iCloud & Internet source calendars — every 5–10 minutes
Sync connections where the source calendar is an iCloud or Internet (.ics URL) calendar sync every 5 to 10 minutes. (Apple does not provide a real-time API that third parties can use.)