When your users follow your subscription calendar, a unique URL for your calendar is stored in the user's calendar client (Google Calendar, Outlook, Exchange, Apple Calendar, etc). That unique URL is regularly requested by that calendar client in order to sync with your subscription calendar. When this happens, any changes you have made to your subscription calendar are synced with your followers' calendars, so their calendars stay up-to-date!
The frequency of this request/sync depends entirely upon the calendar client. Below we have provided the historical sync frequency for each of the major calendar clients: