Cardiovascular activity has been proven to increase protein synthesis in leg muscles, thus increasing your muscle gain, it's by a ridiculous percentage like 20-40%.
It's only less gains if you're not a first year lifter or not in a calorie plus.
So 15 minutes of ropeskipping is enough or isn't it ?
It would have to be incredibly intense rope skipping, such that you would need a break after 30 seconds.
Optimum High intensity should be working you at 80-90%. If you can sprint more than 30 seconds, you're not running fast enough, or you're not running on a high enough incline. It's only truly high intensity if you need a long time to recover, but if it's HIIT you'll notice you'll fatigue the longer you train. Usually HIIT's don't last more than 25 minutes (including rest time).