Keeping fit is enough of a drag without all the confusion over how much, how often, how hard and how long you need to work out. For years the received wisdom was that three 20-minute sessions of vigorous exercise a week was enough to keep us ticking over. But in 1996 the Department of Health decided that 30 minutes of moderate-intensity exercise five times a week was what was required.

Then last week, Tony Blair muddied the waters further, donning his tracksuit to launch the government's Small Change, Big Difference campaign, which advises desk-bound workers to do a minimum of an hour a day, and those with more active jobs (or at least those, such as hairdressing, which involve a lot of standing up) to do some form of physical activity for half an hour daily.

Meanwhile, Sharon Stone has declared that she keeps her perfect 48-year-old figure trim not by endless yoga sessions but simply by walking up the stairs and parking her car further from the shops. So who is right and how much exercise do you really need to do?

Sedentary office workers It makes sense that those who sit on chairs all day need to do more exercise than those who don't. But a whole hour every day? Ideally, yes, say most experts - but that doesn't have to mean a solid hour of pre- or post-work sweating in the gym. The key, says Lee White, a leisure centre manager at Keele University, is to break the time down into manageable 10-minute chunks of moderate activity. White suggests walking briskly to the shops in your lunch hour, stressing that "you should be sweating a little and feeling a rise in temperature". As a guide, the British Heart Foundation (BHF) defines moderate activity as walking a mile in 15 to 17.5 minutes.

Busy workers argue that there is not enough time to fit in an hour's exercise, and convince themselves that exercising harder over a shorter time is just as good. But Len Almond, senior adviser to the BHF, thinks activity sustained over a longer period has more benefit than short, sharp bursts. "It is more valuable in the long-run to be more generally active, rather than get it over and done with in short sessions."

The hour-rule is especially important for those who are generally inactive, according to the American Council on Exercise. But Todd Galati, a research scientist for the council, says you can work slowly towards this goal: "In the meantime, even small bursts of activity can have a great impact on health," he says. A good start in the office would be to walk to the other side of the room to talk to colleagues rather than sending emails, according to Everyday Sport, a Sport England campaign launched last year. And for goodness sake, take the stairs.

Active workers

According to a US study on obesity carried out last year, standing up burns three times as many calories as sitting down over the same period of time. This explains why people with active (or non-desk-based) occupations, such as shop assistants, hairdressers and police officers, do not need to do as much physical activity as others to keep healthy. Depending on the individual, the Department of Health recommends that reasonably active workers should just do around 30 minutes of moderate-intensity exercise at least five days a week. And walking a mile is just as good for you as running the same distance, in terms of building up muscle strength and keeping healthy, according to Everyday Sport.

Builders and labourers rejoice: you can put your feet up after work. The study on which the new Small Change, Big Difference campaign was based, which studied more than 25,000 people, concluded that those with physical jobswere around a third less likely to die from all causes than those who had sedentary jobs and did no physical activity in their spare time.

The same is true for people who do an hour's exercise daily. Regular exercise boosts energy levels, lowers stress and helps to prevent osteoporosis and heart disease. "If you have an active job, then you are doing much more cardio-vascular activity than other people," says Brigitte Swales, an exercise physiologist at Roehampton University.

Children

Despite the national curriculum allocating only two hours of physical education to children each week, the BHF recommends that those aged five to 18 should be doing at least one hour of moderate-intensity activity every day. Getting into good habits at an early age means that a healthy lifestyle will be easier to maintain when they are older. Exercise can include brisk walking, swimming, cycling or simply playing in the playground. Dancing and gymnastics once or twice a week will also improve bone health.

But there are no recommendations for British children under the age of five: a glaring error, according to Almond. In the US, the National Association of Sports and Physical Education advises that children aged two to three should get at least 30 minutes of adultled activity every day, and should have more than one hour a day of free play. When children reach four years old, adults should double the amount of structured activity, such as walking in the park, to one hour a day.