Weekly goals. You don't have to exercise every day, and in fact, you need to let your body recover from time to time. Try 20 - 30 minutes of moderately intense exercise a day, 4 or 5 times each week. Make sure you consult your physician before starting a new exercise regiment.
Depends. How old are you? How is your health in general? Are you overweight? If so, how much? All that goes into your "plan" for better health through exercise! For healthy, 20-30 min of moderate aerobic exercise every day is the goal. If you're a kid (under 25) you may need more. If you have diabetes or heart disease you should consult a doctor before you start an exercise program. Hope that helps.
Varied opinions. Depending on your overall health, medical condition and need for exercise, even as much as 10 minutes of any kind of exercise 3-7 times a week can make a difference in your health. Certainly the more fit one is, the more exercise can be done. I would recommend 30-45 minutes 3-4 times a week and more if can, to keep you at your best health. Speak with your pcp, personal trainer or qualified prof.