This workout is one of my most favorite to train, as it can really develop the endurance you look for when you are in a fight, tournament, or sparring match.  I want to high stress that you work at your own pace.  Don’t try so hard that you find yourself waking up on the floor, or worse yet, in a hospital.   Use common sense, and if you don’t have any, please borrow some from a friend. 
You should do this workout based on your skill level as well as your current endurance.
Beginners should start at 2 minutes, which is typical of a sport karate tournament.�
Intermedate fighters should start at a 3 minutes.
Advanced or seasoned fighters should start at 5 minutes. 
You can do this training exercise with a partner, or go solo with a heavy bag, or free standing dummy.
During the first round, start out by using hand and elbow techniques continuously, and try not to stop hitting.  You can make brief rests as you would in sparring, but don’t break for so long, you lose out on the value of the workout.  Rest for 30 seconds or a minute, depending on your endurance level.
Onto the second round, practice only kicking and kneeing the bag or shield.  Work on moving around the target as if you’re sparring, but don’t move around so much you waste too much time not kicking. 
Finally, the last round, incorporate your punching and kicking techniques throughout the duration of the round. 
This is a good workout if you don’t have a lot of time, but want to get an intense workout in with short amount of time.  That is pretty much a 15 minute workout.