typically around 39/13 to 39/15. Whatever I can hold the rpm with. Sometimes I end up dropping to a lower gear to keep the cadence up when I'm dying. (I've found that its easier to do on the small chain ring because 39/13 is more than big enough for me, and its easier to bale to a lower gear if you just have to shift the rear)
My typical speed is pretty disappointing doing them. I tell myself its because they're designed be inefficient from a power/ speed point of view to make your heart rate soar; either that or I just suck.