First time I do them I'm good for only a couple of minutes before I'm spent.
By the end of the winter this past year, 400/250 for 5min, 300 for 5min, 400/250 for 5min was a pretty standard workout. 15s on/off for the 400/250. Pretty miserable.
I have some sort of mental block that prevents me from really killing myself indoors, though.