I think it varies from person to person, depending on the intensity. I dedicate 20 minutes a day. I run at an average 10 kilometers per hour. It is pretty intense for me as after, I feel so energized and that something great has just taken place. I am so soaked in sweat and it feels really good.
The length of time to exercise depends a bit on the activity. I jog for about 30 minutes and it is enough for me. If I am cycling, one to two hours is normal for me as it is less intense. Working out usually takes me about 45 minutes. I think the important thing is to dedicate the time. Less important is the amount. Once you have started your exercising you will likely follow through. I would suggest thirty minutes to an hour a day would be a good length of time for the average person doing moderate activity.
You don't need to exercise everyday, 3 or 4 times a week is enough. However, if you want to exercise everyday that is definitely fine and will be great for your health! Exercising 30 minutes a day, if you do it everyday, is enough to be healthy. I would do at least 1 hour a day just because if my body is already warmed up and ready for a workout I might as well just give it at least 1 hour of effort! But 30 minutes should be good enough!
Depends on what you're trying to do with your fitness. If you just want to stay healthy and in relatively good shape, then 20-30 minutes a day of some activity is fine. If you're looking to get in killer shape and drastically change your body tone, then you're looking at dedicating a much larger portion of your life to working out, eating correctly, counting calories, etc. For me, I try and do 30 minutes/day at least 5 times a week. Especially during the semester, it's difficult for me to allocate more time than that to working out.
Yep. Your fitness goals should be what you use to decide how long and the way you work out. Jogging for example for ten minutes for me wouldn't be enough. I always go an hour or so, one way. That's just me though, if 20 minutes is OK for you and you're fit that's good. But you also need to put in a little more like doing some push ups, etc when you're done running.
Regardless of how long you work out, it shouldn't be until you get burned out. Like you said, it varies from person to person depending on the person's physique. Beginners should go with slow and light exercises and work up from there. Once we get comfortable we can then increase the intensity.
It depends what you're trying to achieve, but I would say generally at least 30 minutes is good. Even if you're just walking for 20 minutes and stretching out for 10! I think it's good to be active at least 30 minutes a day a couple of days a week. I know a lot of people who aren't.