It depends on the intensity and your body weight. Age matters too as the metabolism of youth ranging from 15 to 25 are significantly higher compared to the people age 30 and above.
If you are...
It depends on the instructor. I wear a heart rate monitor at the gym and in a 45 minute class depending on the instructor I can burn 300-475 calories. It also varies on how hard you push yourself.
hum... two answers here. 10 cal for 20 min and 200 calories for 30 min. Think I will put in the effort for that extra 10 min. :). Just kidding- i dont know which is correct.