There have been some several recommendation statements issued recently regarding recommended physical activity. The following is a summary of what is further expounded upon below:

• ACSM: Moderate-intensity cardiorespiratory exercise training for 30 minutes or more on 5 days a week (150+ minutes week), vigorous-intensity cardiorespiratory exercise training for 20 minutes or more on 3 days a week (75+ minutes total per week), or a combination of moderate- and vigorous-intensity exercise to achieve a total energy expenditure of 500-1000 MET-minute each week.
• US Dept HHS: 2 hours and 30 minutes a week of moderate-intensity aerobic exercise, or 1 hour and 15 minutes (75 minutes) a week of vigorous-intensity. Muscle-strengthening activities that involve all major muscle groups performed on 2 or more days a week (outlined very well here)
• USDA: moderate physical activity at least 30 minutes per day. Up to 60 minutes of moderate to vigorous physical activity on most days to prevent weight gain, and 60 to 90 minutes of moderate physical activity daily to help avoid regain of weight.
• ACSM: at least 150 min/wk of moderate-intensity for weight loss and prevention of regain.

In 1996, The United States Surgeon General recommended the following in "Physical Activity & Health: A Report of the Surgeon General" where the Major Conclusions state:

Significant health benefits can be obtained by including a moderate amount of physical activity (e.g., 30 minutes of brisk walking or raking leaves, 15 minutes of running, or 45 minutes of playing volleyball) on most, if not all, days of the week. Through a modest increase in daily activity, most Americans can improve their health and quality of life.
Additional health benefits can be gained through greater amounts of physical activity. People who can maintain a regular regimen of activity that is of longer duration or of more vigorous intensity are likely to derive greater benefit.

In 2011, the ACSM issued a position stand Quantity and Quality of Exercise for Developing and Maintaining Cardiorespiratory, Musculoskeletal, and Neuromotor Fitness in Apparently Healthy Adults: Guidance for Prescribing Exercise recommending:

Moderate-intensity cardiorespiratory exercise training for 30 minutes or more on 5 days a week (150+ minutes week), vigorous-intensity cardiorespiratory exercise training for 20 minutes or more on 3 days a week (75+ minutes total per week), or a combination of moderate- and vigorous-intensity exercise to achieve a total energy expenditure of 500-1000 MET-minute each week. On 2-3 days per week, adults should also perform resistance exercises for each of the major muscle groups, and neuromotor exercise involving balance, agility, and coordination. Crucial to maintaining joint range of movement, completing a series of flexibility exercises for each the major muscle-tendon groups (a total of 60 seconds per exercise) on 2 or more days per week is recommended. 

A well-organized table of the evidence-based recommendations for the different components is also available

The United States Department of Agriculture (USDA) published Nutrition and Your Health: Dietary Guidelines for Americans in 1995, recommending:

Making moderate physical activity a part of an adult's daily routine for at least 30 minutes per day promotes fitness and reduces the risk of chronic health conditions such as obesity, hypertension, diabetes, and coronary artery disease. (Walking at a brisk pace [3 to 4 miles per hour] is an example of a moderate physical activity.) Moderate physical activity for an hour each day can increase energy expenditure by about 150 to 200 calories, depending on body size. If not offset by increased calorie intake, this increase in physical activity could be helpful in preventing weight gain. Many adults need to participate in up to 60 minutes of moderate to vigorous physical activity on most days to prevent unhealthy weight gain, while adults who have previously lost weight may need 60 to 90 minutes of moderate physical activity daily to help avoid regain of weight. Children and adolescents need at least 60 minutes of moderate to vigorous physical activity on most days for the maintenance of good health and fitness and for healthy weight gain during growth. Compared with moderate physical activity, vigorous physical activity provides greater benefits for physical fitness and burns more calories per unit time.

In 2007 a position statement issued jointly by the American College of Sports Medicine (ACSM) and the American Heart Association (AHA) titled "Physical Activity and Public Health: Updated Recommendation for Adults" (see PDF ), recommends:

To promote and maintain health, all healthy adults aged 18 to 65 yr need moderate-intensity aerobic (endurance) physical activity for a minimum of 30 min on five days each week or vigorous-intensity aerobic physical activity for a minimum of 20 min on three days each week. [I (A)] Combinations of moderate- and vigorous-intensity activity can be performed to meet this recommendation. [IIa (B)] For example, a person can meet the recommendation by walking briskly for 30 min twice during the week and then jogging for 20 min on two other days. Moderate-intensity aerobic activity, which is generally equivalent to a brisk walk and noticeably accelerates the heart rate, can be accumulated toward the 30-min minimum by performing bouts each lasting 10 or more minutes. [I (B)] Vigorous-intensity activity is exemplified by jogging, and causes rapid breathing and a substantial increase in heart rate. In addition, every adult should perform activities that maintain or increase muscular strength and endurance a minimum of two days each week. [IIa (A)] Because of the dose-response relation between physical activity and health, persons who wish to further improve their personal fitness, reduce their risk for chronic diseases and disabilities or prevent unhealthy weight gain may benefit by exceeding the minimum recommended amounts of physical activity. [I (A)]

The "2008 Physical Activity Guidelines for Americans" presented by the The US Department of Health & Human Services recommends:

Adults should do 2 hours and 30 minutes a week of moderate-intensity, or 1 hour and 15 minutes (75 minutes) a week of vigorous-intensity aerobic physical activity, or an equivalent combination of moderate- and vigorous-intensity aerobic physical activity. Aerobic activity should be performed in episodes of at least 10 minutes, preferably spread throughout the week.

Additional health benefits are provided by increasing to 5 hours (300 minutes) a week of moderate-intensity aerobic physical activity, or 2 hours and 30 minutes a week of vigorous-intensity physical activity, or an equivalent combination of both.

Adults should also do muscle-strengthening activities that involve all major muscle groups performed on 2 or more days per week.

Regarding weight loss, in 2009 the ACSM issued a statement in "Appropriate Physical Activity Intervention Strategies for Weight Loss and Prevention of Weight Regain for Adults" and emphasized that this was consistent with the 2008 guidelines from the  US Department of Health and Human services. The ACSM statement says:

On the basis of the available scientific literature, the ACSM recommends that adults participate in at least 150 min/wk of moderate-intensity PA to prevent significant weight gain and reduce associated chronic disease risk factors. It is recommended that overweight and obese individuals participate in this level of PA to elicit modest reductions in body weight. However, there is likely a dose effect of PA, with greater weight loss and enhanced prevention of weight regained with doses of PA that approximate 250 to 300 min/wk (approximately 2000 kcal/wk) of moderate intensity PA.

The changes in physical activity recommendations over the last several years have been rather subtle. There will likely be changes to this paradigm as new knowledge is gained. However, the guidelines presented above are--aside from some minor difference--mutually supportive and represent good objective recommendations.