From what I hear (from my brothers girl friend) is that INS will sit on applications for ever unless you get an immigration attorney. After she did that, her green card was delivered within 3 months.
I can’t tell you when you will get a reply from INS, I can tell you that I had a similar experience. It took over two years (AFTER my interview that I passed) before I was contacted to attend a naturalization ceremony. I am Moroccan and was told the same thing as you, that it was the FBI check they were waiting for. I contacted ACLU and my local representatives who made contacts with INS and FBI on my behalf. I don’t really know whether it helped or not but I did finally swear in and get my citizenship. There is hope, but you may need to push them. INS is all politics, so get your representatives involved. On the other hand, if you know you have a criminal history anywhere in the world or have been suspected of any crime, you may wait for a very long time, much longer than you have already.
I say, contact outside organizations that are there to help you, like ACLU (American Civil Liberties Union) or a State Rep., even an attorney (make sure you get a well known Immigration attorney if you do this). But keep your hopes up and keep up the inquiries!