Skipping is quite high intensity thing. It is equal to running @12 km/hr. 20 minutes of skipping = close to 250 calories. If sometime you get bored of gymming and running, you can do skipping for a change. Its easy, inexpensive, can be done at home and doesn't require any major accessories.
I dont know if there is any maximum anyone can do. It will all depends on person to person, but I have never been able to do it more than 2-3 minutes continuously. I do close to 80-100 in a minute.