I believe they generally advise that you're likely to be in the Embassy for about 3 hours or so.
I think it's similar to how they book visa interview appointments in that they will book dozens of appointments, all scheduled for 0830 in the morning and then when you get inside, you're given a number and you just have to sit around inside and wait until they get to you.
I've been to the US Embassy for visa interviews 4 times now and each time, I've been in there for 3-4 hours :S. The actual interview part only took 3 minutes... the other 3-4 hours were spent sitting around, reading a book and waiting for my number to be called.
This is what the US Embassy website says about passport interviews and reports of birth abroad:
Wait Times for Passport Interviews
Wait times for passport interviews may range from 2 - 3 hours depending on the number of customers. Please be aware that wait times are also longer for first time applicants and Consular Reports of Birth Abroad. Generally speaking, the earlier your appointment time, the shorter your wait.