Assuming you get an appointment, I'd allow more than a couple of hours.
Last time I was home in the US I had to get a replacement passport in Boston. The procedure was to go in at the appointment time, give the agent the app/photo/payment, and come back between 4pm and 430pm when the pick up window was open. So even though I had a 930am appointment, I didn't have a passport in hand until later in the day.
I got the impression this was standard procedure, and I'd assume it applies for all offices.