You should plan to start your exam at the time of your scheduled appointment. The "start exam" link will be displayed to you as early as 15 minutes prior to your appointment, but the proctor may not be available until the appointment start time. The link is displayed prior to the appointment so you can ensure the proper functioning of your computer prior to taking the exam.
The "start exam" link will remain available to you for 15 minutes past your scheduled appointment. This leeway is provided in case you require some extra time to sort out technical problems that you may encounter.
In sum, you have a 30-minute window for starting your exam but you should plan to start at the time of your scheduled appointment.