Actually, even the elite runner athletes only do at most 1-2 HIIT sprint sessions per week (unless they are a sprinter ofcourse). I believe I read that in one of Jack Daniels' (the famous running coach) books. And, not only do they only do it 1-2x's a week, they also only do it through certain periods of their training. So, the take home message there would be......1-2x's a week of sprints and then some steady state cardio and you are doing alright. Also, don't be afraid to take a break from doing sprints all the time in your program. Cycle them in...if you have been hitting it hard on the sprints for the past 6 weeks, give your body a break for 2-3 weeks and just go with some regular cardio. Like everything, work it in, work it out, and periodize.
It is a lot of work on the body to do sprints. It ranks right up there with leg training as far as the amount of work you are doing (tissue turnover) and neurologically, it is very intense as well. So, if you are training legs hard, that is another reason to take it easy on the interval sprints so that you can recover between workouts. The steady state cardio will help not only to maintain your current cardiovascular state (or improve it if you are charting things properly) but it will also aid in some active rest. This is one reason I don't like HIIT so much when bulking. Eat more food, increase your lower body training volume and drop back to some steady state cardio so that you don't (a) burn your legs out, (b) can maintain some cardiovascular health without burning up all those calories you are eating and (c) you give yourself a break from the sprints. Then, after the bulking cycle, you add the sprints back in to increase caloric burn, decrease leg training volume so that you don't overtrain them in a hypocaloric state and the sprints are new and fresh again since you haven't been doing them.
