Depends on the type the intervals intensity and overall your endurance. The shorter your rest periods are and the longer your H.I.T the more calories you will burn per minute. When I was boxing we did up to 45 minute sessions. But with lifting weights I usually keep it around 20 minutes
As long as I can , no but seriously .. If I do tabata style I like to keep it nice and short .. Oh amount of time I have in the day to squeeze it in usually determines the intensity style I do as well
When I was getting ready for my last show i was advised to do: 45 sec running/15 jogging. I did it once and my knees were going to give out. So I did 1 min biking fast/1 min slow, 30 intervals.