I'm 24 so I get my heart rate up to 191 but that's on the last set. I wait till my heart rate is down to 145 before I do the next set. Usually it is between a minute and a half to 2 minutes. I set the bike to max tension and pedal as fast as possible for 1 minute. I do this 8 times. If you can't do max tension than just use trial and error to figure out something you can go fast on but not too fast on. To figure out what your heart rate max is subtract your age by 220 so my max is 196.