i don't know about intensity or time... i take a circuit class where we mix up cardio and strength training once a week... it's my favorite class. she mixes it up so its never the same from week-to-week. during the cardio, she tells us to up it our last 30-45 seconds, depending on the length of what we were doing. i do know i use only my heaviest weights in that class as there are less reps than body sculpting, where i use a lighter weight and heavier weight....