What if your heartbeat is 100?

100 beats per minute, during aerobic exercise, is fine. Depending on your health, this can be maintained for various periods of time: a half-hour would be a pretty long period of exercise but not excessive.

However, if your heartbeat is 100 beats per minute when you're resting, you'll probably want to get a health checkup. At rest, the normal heartbeat is approximately 72 beats per minute on average.