Watch TurboFire: HIIT 15 (2010) : Full Movie Online Free This is the shortest HIIT workout at 15 minutes long. It may seem like the longest 15 minutes of your life at first but, after doing this workout several times, it gets easier. You will go through the one minute HIIT intervals repeatedly throughout this workout and will not believe how high you can get your heart rate in only 15 minutes.