I guess you have to listen to your body. I usually try and rest after six days, but have done seven or more before. It kind of depends on how you feel. I know when I was getting ready for my class reunion last August, I was doing heavy cardio (incorporated with strength workouts) 6 days (or more at times) and sometimes my body said I was tired after the 5th or 6th day. For cardio, I was doing big workouts like the Imaxes, Rhythmic Step, Step Blast, Kick Punch and Crunch, etc. Plus I was incorporating heavy weight training as well (Gym Styles, Pyramids, etc.) You really do have to listen to your body (like Cathe has said but I don't always do!). Don't overdo it. After doing both the wrong and right ways, the right way is better, in my experience!
Julie