It should only take no longer than 1 hour, my wife had her done and it depends what kind of cermony. Some can take 5 mins others a little bit longer but I never heard anyone who was more than 1 hour.
They would save so much time and money if they just had you go to any courthouse and take the oath in front of a judge, get a receipt of that, and send it in for your naturalisation certificate, no? Isn't that a possibility?