I think it depends on what u do. I wouldn't do 2 hard days in a row. I always take 1 day off a week. I don't know what u do for cardio but I wouldn't do anything super hard for 2 days in a row. If u go to the gym and do cardio and stay until u can barely walk out alive, then don't do that much the next day. Give it a day or 2. But I usually don't like to totally just do nothing on a day after a hard workout. I go for a light jog or something - just to keep from feeling all sore and stiff the next day. But if u go to the gym and just get a good, normal, 25-35 minutes of steady cardio ... u can probably go almost every day. I would always take at least 1 day off a week. Just listen to your body. Your body always knows when it wants more or less.