I usually go into the anaerobic zone for 20 minutes (90% or more of my Maximal heart rate), but it's a bitch of a workout, so lately I've been staying mostly in the aerobic range, but I noticed that I'm losing a little strength ( number of reps with same weight) and intensity and I think it might be related to backing off on the intensity of the cardio workouts. Short intense cardio workouts have always enabled me to do more reps on high rep workouts such as legs.