There is no long wait so to speak. I had my interview on April 19th. Once you get to the office on LaSalle Street you will have to go through Security and place your things through the scanner. Make sure you have your letter and some form of ID. After you check in you will go to the waiting room which is a large room.
Sit and wait for them to call your name which might be after the schedule time for your interview. Once your name is called you will follow the person to the booth where he or she will conduct your interview. First you will be ask to raise your right hand and swear to tell the truth and nothing but the truth. You will be aske 10 questions but if you get the 1st 6 right that is it. You will also be ask to write one sentence in english.
Once you pass the test they will go over your application and ask you questions from it. The whole procedure takes about 15-20 minutes. You will get a sheet of paper that tell you if you pass or fail.
You will get your date for the oath in the mail which is schedule one month from the date of testing.
I got my US Citizenship on May 19th and applied for my passport.
Good luck on your interview. Would be happy to hear back from you.