It depends on how much you train normally and what your goals are. If you're burned out, I would assume you've been training for a very long time without a break and so I would take a week off from doing all exercise. Give your body a complete break to let it restore itself. When you're doing cardio, you're still using your lower body muscles as well as your back. Just as people do periodic fasting to give their intestinal track a break, you should fast from exercise to give your muscles a break. A lot of professional bodybuilders take a week off every few months and some even take a whole month off every few years. When you go back, you will be stronger and might push through any plateaus you might have. In cardio, however, you will still be using your muscles so it's not a complete break. Also, there's a danger of cardio and no weight training cutting into your muscle mass. Especially if you do cardio every day for a long period of time. You may feel even more burned out after a week of doing that and I have found that too much cardio burns more muscle than fat on me so it's not really worth it. Every body is different, though.
Basically, I would just take a week off from ALL exercise. Others may and probably will disagree.