When a new appointment is added to your schedule (from either the client scheduling or from you adding the appointment as a staff person) you'll notice that the email reminders default to the same value each time. You can change this default value as it applies to appointments by going to the Appointments → More Actions dropdown and choosing the option for "Appointment Defaults":


On the window that appears after you click "Appointment Actions" you can change the default number of hours that the reminder email will be set to from the top two dropdowns. Once you've finished your selection, you can press "Save Changes" at the bottom of the window to commit the new default settings:


You can also select to not send the reminder email from here: