What exactly are you goals from the cardio, is it to burn fat or just for health? I like to do 20 minutes of low-intensity cardio after a workout or HIIT on off days, but if you can find the energy, go ahead and do some HIIT. I would strongly suggest getting in some BCAA's/EAA's etc. right before you do the HIIT to minimize any muscle wasting, and have a shake right after you finish.