It lasted two hours. Now the way it worked was that I was doing on my own but I had to constantly explain to a "peer" my line of reasoning. So I was not alone in front of a white paper :-) This was an extremely interesting wy to interview. I felt I had the opportunity to demonstrate my skills on a difficult problem (which I didn't solve). I didn't remember the details of Dijkstra's algorithm but I had the opportunity to show that I can approach a complex problem in a logical and efficient way (well, that day, that kind of problem; don't ask me to work on helicoidal tomography :-))