In CrystalEMR, the standard time-slot is of 15 minutes.
It will be practical to give appointments in interval of 15 minutes (i.e. 12:00 PM, 12:15 PM, 12:30 PM, 12:45 PM etc.). So that it will be easy for any patient to remember the appointment time.
Refer following article to create time-slot of consultant:
Can I define my own time-slots? Also Can I book more than 1 patient per time-slot on my own?