One of the first questions people ask when they're wanting to get fit is "how much exercise is good for me?". So whether you are on a strict exercise plan that a trainer designed or doing it for yourself, "how much exercise" is quite a subjective matter. In reality, these people are often actually asking, how little do they need to do? The answer, like the length of the proverbial piece of string, depends on circumstance.
One way to determine how much exercise you need is to talk to a professional. If you have been inactive for a long time, have health problems, or are elderly or pregnant, then you should consult your doctor. He will have suggestions as to how much exercise is good for you.
Alternatively, you can consult a personal trainer. They will talk to you about your health and fitness history, and your work-out goals, and then come up with a programme that answers all your questions.
However, if you are working out on your own, then there are certain basic guidelines you can stick to in figuring out you work-out programme. It is best to begin carefully and build up - this will allow your body time to adapt, and also give you the reinforcement of making your goals to start with. It also allows time for you to figure out the balance of your life when you are having to make extra time for your workouts.
If you start with three days per week, for 30 to 60 minutes at a time, you will be doing enough to maintain general good health, but not overdoing it. If your goal is to get fitter, or lose weight, for example, then increasing your workload will allow you to make progress more quickly.
As your body adapts to the workouts you can increase the frequency, length, and intensity. But remember to always allow one or two days of rest a week in which your body has the opportunity to recover and grow.