Just been up there today. It took me about 4 hours from arriving outside to being done. Sadly I didn't get the visa (M1) though because my passport was too tatty. You don't want any damage anywhere near the data strip on the bottom of the photo page!
So.. new passport on monday! I don't have to go back... the US Embassy will pick up by courier, process it and return it to me apparently. I was told the turnaround time would be no more than a week once they have it back.
TSA approval took me about 4 weeks should you be wondering about that.