Twelve minutes of aerobic exercise like, ski machine, swimming, cycling or, if you have to, running. (Running is the least healthy as it cracks up your ankles/knees/hips in later life and has your heart bouncing up and down inside your chest).
That is twelve minutes continuous when the heart rate has reached the aerobic level, so seventeen to twenty minutes should do it, anything after that twelve minutes is only burning calories and therefore good for weight loss, the cardiac work has ben done. Stop/Start games like tennis, squash, badminton etc. are healthy but you really do need that continuous twelve minutes at the aerobic level. Got all this from a heart specialist, by the way.
They say eating oatmeal, like porridge etc. can reduce cholesterol too.