Baldie,
if it is the M-1 visa you are waiting for it took about three and a bit weeks. I rang the £1.90 a minute phone line towards the end and they told me that the visa applications are processed in day blocks, ie if there is a twenty day wait all apps sent in today WILL be processed on the twentieth working day.
Again my problems with the US INS began with a two hour wait at the airport of entry whilst they processed me. This was due to the fact that whilst I had my visa pasted into my passport, I did'nt have the poxy yellow piece of paper (I-10). I thought that this was only the visa application form and now that I had visa it was defunct. There was no information from the London embassy stating that this paper was important. (It is hidden in very small print on the back of the form). SO MAKE SURE YOU TAKE THE YELLOW PIECE OF PAPER!
Apart from that expected to be pulled apart about twice on the way in and out of airoprts over there.
good luck
Gaz