I'm 56 and I've found that once a week is good enough for Interval training. If you're younger it may be beneficial to do it more often. What I do is warmup for 10 minutes and then do 30 seconds as hard as I can and 30 seconds recovery. I do 10 reps. By rep 7 I'm getting in the 95% max heart rate. Rep 8,9 10 is pure misery but you have to pay close attention injury avoidance. I then do 10 minutes of warm down at a slower than warm up rate. So while my interval workout is 30 minutes long only 10 minutes is doing intervals and of those 10 minutes only 5 are high intensity. My other workouts are are typically in the < 70% max heartrate.