在西雅图入籍面试到宣誓等多长时间呀

I live in Seattle, submitted N-400 US natualization Jan. of 2009, and did Finger print Feb. (one and half months later RD), 2 weeks later received notice for interview, and schedule on early of May, 2009. My question is who knows after interview, how long I need to wait for oath ceremony to complete all my application process? Some people said I could get the oath ceremony on the same day of interiew, some said I have to wait for mail notice from USCIS, and the waiting time is unknown.
Who has the experience in Seattle area just finished N-400 case recently? Many thanks for your kind response or info.