Shortening Intervals Today we begin shortening our interval period and also shortening the rest period. We're shooting for 45 seconds of high-intensity exercise followed by a 30 second break. Those new to interval training may need up to 45 seconds to recover before starting the next period. Those new to cardio or interval training should start with 15-30 minutes of a solid cardio before attempting intervals. As with most interval days, feel free to substitute biking, elliptical, swimming, or even rowing for the running.