According to the American College of Sports Medicine (ACSM), a HIIT workout typically lasts 20 to 60 minutes in total, counting the alternating hard work periods and easier recovery periods.
Each work interval can be as short as 5 seconds or as long as 8 minutes, performed at 80% to 95% of your estimated maximum heart rate, so it feels hard to very hard. Recovery periods are often as long as the work periods and are done at 40% to 50% of maximum heart rate. A common format is a 1:1 ratio, such as 3, 4 or 5 minutes hard followed by the same time easy. Sprint interval training is shorter: about 30 seconds of near all-out effort and 4 to 4.5 minutes of recovery, repeated 3 to 5 times. ACSM suggests starting with one HIIT session a week, adding a second when ready, and building a base of regular aerobic exercise first. People who have been inactive or have heart risk factors may need medical clearance before starting.
Source: American College of Sports Medicine, ACSM Information On High-Intensity Interval Training (brochure)
Image: Indoor cycling class, U.S. Air Force photo by Airman 1st Class Madeline Herzog, public domain, via Wikimedia Commons.