There is a time limit of 30 days from conditional approval to make the appointment but the appointment does not need to be within the 30 days. This is the extract from my conditional approval notification:
'In order to finalize your Global Entry enrollment, you must schedule your Global Entry Interview within 30 days of the date of this letter. You do not have to complete the interview within 30 days'.

You can also change the appointment up to 24 hours before the appointment.