With an increase in the intensity of your intervals, the duration of each interval and the total time spent doing the intervals should decrease. It is the same concept as weight lifting where the more the weight the fewer the reps one has to do. On the bike, when using greater force, the more intense the level of intensity in a single workout, the shorter the time spent at this intensity. It is critical to maintain a proper ratio of intensity versus duration so that the workout can be done effectively without the risk of overtraining or reaching the point of burnt out.