To get any part of your body to be a little stronger and work a little more efficiently, you have to challenge it to push past its usual working capacity. Aerobic activities push the heart, thereby increasing its pumping abilities and helping to keep heart disease at bay.
At a minimum, you should try to engage in aerobic activities three days a week, for 30 minutes at a time. (You can break it up into three 10-minute sessions or two 15-minute sessions at first.) Optimally, you'll work up to five or six days a week, for 30-plus minutes each session.