When you create a new Outlook appointment, Outlook sets itself a reminder to remind you (with a pop-up message) 15 minutes before the actual appointment. It is easy to change the 15 minutes on individual appointments or to change the default to a more suitable period.
To change the reminder on an individual appointment:
Open the Appointment.
Change the time shown in the Reminder dropdown.
To change the default reminder time:
If you find yourself changing the reminder times on individual appointments, it may be easier to simply change the default from 15 minutes.
In Outlook 2010:
In Outlook 2000/2002/2003/2007: