It starts the second you are allowed to access information, i.e. when you start reading the patient information, vitals and chief complaint. Most people from what I've seen take 30-60 seconds outside processing the information, creating a template for interview on their blank sheet, etc. I usually liked to come up with a 3-item differential as well, esp. if I thought it might be abuse or depression or some information that may need a little coaxing to unveil. Sure, you lose that time from your interview but that way you can make sure you don't forget to ask anything and that you have some sense of direction going in. You don't want to conduct the whole interview and then walk out of there only to realize you forgot to ask about Family History or allergies... it may be irrelevant to your diagnosis but it still needs to be asked for completeness' sake.