The 120 Rule for Cooling Down
How long should a cool-down be? Experts say that you should not stop continuous movement until your heart rate is below 120 beats per minute. If you've had heart problems of if you're taking medication that slows the heart rate, you may need to cool down to a lower heart rate; ask your physician or trainer for guidance.
One more thing: Remember that cooling down and stretching are not the same - but they're both very important. Cool down first, and then do your stretches.