I do my cardio at noon time , three hours after my protein shake. Morning first thing would probably be better but my stomach feels pretty empty at that point(noon). If you are like me and only work one body part a day, then do cardio every day(well 5-6 days/wk), if you do an eod workout that has you in the gym for an hour or so than I would skip cardio on workout days since your workout will suffice since it is likely an interval training situation.