On your E-mail Settings > Follow-up page, you can set how many days after an appointment follow-up notifications are sent out. It can be sent anywhere from 1 to 365 days after a client's appointment. You can also enter fractional days; for example, enter 0.25 to send follow-ups 6 hours after the appointment.
The follow-ups are sent the same time of day as the appointment. So, if you have follow-up notifications set to be sent out 1 day after an appointment, and someone scheduled an appointment for Tuesday at 1:00pm, the notification would be sent out on Wednesday at 1:00pm.
A follow-up email is sent out after each class time within a class series as well!