Appointment Reminder syncs with Google Calendar in real time. So if you add an appointment in Google Calendar it will show up in Appointment Reminder almost straight away. There can sometimes be some slight delays but should show up within a couple minutes at the most.
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If you go to Calendar > Edit Calendars you can see when the calendar was last synced.

Not Showing?
Try refreshing the page to see if the appointment is now showing in the calendar.  If it's still not there then please contact support and will help you figure out why.