I think jump roping until you trip, your calves get tired, or your heart/lungs get tired is safe. There are very few health risks to jump roping:
1. It is possible that you could injure your ankle joints from excessive jumping, but the damage here is insignificant and is way less than the joint shock from running.
2. If you over train yourself to the EXTREME, you could push your heart past its maximum beats per minute and you could go into cardiac arrest.
I jump rope a lot and its only helped me, so I don't see how it could hurt you.
