A typical HIIT workout usually lasts about 20-30 minutes and involves a 2:1 ratio, meaning your recovery intervals are twice as long as the work intervals. An example would be 5-10 high intensity sprints (working at a Level 8-9 on this perceived exertion chart) lasting 30-60 seconds interspersed with recovery intervals of 1-2 minutes (working at a Level 4-5). Examples of interval workouts.
Talanian,J; Galloway, S; Heigenhauser, G; et al. Two weeks of high-intensity aerobic interval training increases the capacity for fat oxidation during exercise in women. J Appl Physiol 102: 1439-1447, 2007.