To get the best out of your cardio workout you should work towards either your intensity level or use your heart rate as an indicator of what benefit you are getting from the excercise.
There are three important ranges of heart rate for different types of excercise:
Instructions to Calculate your Heart Rate Bands
Enter your age and resting heart-rate and then select your gender.
How the Bands are calculated
(Males: 214-(0.8*age). Females: 209-(0.7*age)). Press "Calculate".
The training zone values are calculated using the Karvonen formula:
Heartrate = ((Max hr-Resting hr)*%X/100) Resting hr. (where %X ="%MAX," e.g. 60)