The minimum recommended by both the US Department of Health and the UK government is 30 minutes of moderate exercise 5 days a week. Moderate should mean that your breathing speeds up and you feel warm, possibly sweating a little. A brisk walk counts as moderate exercise, as do more energetic forms of work such as digging the garden. You can also do your exercise in short sections if you prefer - for instance, if you take a fairly fast walk for 15 minutes morning and evening, that would be the minimum needed.

This much exercise should mean that your heart and other organs remain healthy, that your muscles work and, unless you are overeating or having a lot of high calorie food, that you don't become seriously overweight. However, for really good quality of life you need a bit more exercise - adding on an hour of aerobics or something else challenging is a good idea - and of course a moderate food intake.