Generally, three sessions of aerobic exercise per week, with each lasting 20 to 30 minutes, should suffice. A vigorous walk, swimming, riding a bike, or anything similar counts as aerobic exercise. You just need to get your heart pumping along within your target range for aerobic exercise. This is a number that is based on your age, but generally is between 100 and 150 beats per minute (bpm).
As the guy above stated its generally three sessions a week, as they say that when you excersie your breaking muscular tissue in order to create new and stronger ones. if you excersie to much youw ill have to rest up for a long time in order to excersise again. i learned this the hard way and it was pretty demoralising and near put me off excersise.
for a good heart rate excersise try very basic ones jogging or a long walk (again as stated above sorry danny9999 ^-^;) but never over strain youreself friend and best of luck!. oh an odd side note dont forget tomatoes are your hearts best friend :)