Most people need to exercise about seven hours a week.  The US Government offers these exercise guidelines, which differ by segment of the population:

• Adults, to prevent weight gain:  30 minutes or more of moderate activity on most days, although 60 minutes of moderate to vigorous activity on most days is better
• Adults, to lose weight:  60 – 90 minutes per day of moderate to vigorous activity  
• Adolescents and children:  60 minutes or more per day

In addition, adults need strength-training exercise two days a week to work all major muscle groups.