20 minutes of cardio per day is recommended when you lift weights. If it is your rest day, you can do up to 40 minutes cardio. It depends if you body can handle it.
If you go on a cardio machine for 60 minutes a day, you're at less risk for a heart attack, and other health defects AND you burn 600 calories (on average). This helps you in the short run and in the long run -- you look great, loose weight and get become more healthy.
This is better than skipping rope for 20 minutes everyday.
If you have a cardiac problem, are an older person such as 35 and up or you have not exercised for a very long time, then you should ask your doctor.
If you do exercise often, young and in good health I think you should build yourself up every day little by little till you reach the 20 minuets.. Your heart rate will tell you, as you exercise how long should jump rope.