I personally don't think you should exercise for a set amount of time. I think it's better to listen to your body while you move. Exercise until your body is tired, and then push a little further. Listen to your mind also. Take note of your feelings and emotions. Exercise should help clear your mind, so I find that pushing myself until I truely feel satisfied is better than setting an arbitrary time limit. But if limits help you, start with walking for 10 minutes a day (or whatever time you think you can manage) and work your way up from there. Starting at 10 minutes or 10 reps is manageable, expecting yourself to be able to do an hour or a hundred reps right at the start is setting you up for exhaustion.
The thing about should varies from one person to another. I say 15 mins is the best amount for a workout. But if you really really think about it, give it 7 minutes. You will thoroughly enjoy your workout