This is my most frequently asked question both by users of the gym and new runners or those coming back from a break.  If you have had  a break from exercise whether it is 6 years or 6 weeks or in fact, never exercised at all, the answer is nearly always 6 weeks.
For new gym users this would normally be six weeks of light aerobic exercise where I would tutor you in the level you need to be exercising at.  We would also learn a few stretches and dynamic exercises.  After the six weeks you will notice that your legs are stronger and that you will get less out of breath doing normal activities and most people tell me that they have more energy.  Your heart muscle will get stronger and the blood supply to your muscles will improve and on top of this your muscle chemistry will improve.  Some people will lose weight , for others it will take longer (but inches will be lost form your middle if you have one!).
For the runners the first six weeks should also be easy running at a nice comfortable conversational pace.  Teaching people that they don’t have to go fast to get fitter is probably what I spend most time tutoring.
It is the time that you are exercising for, not how hard you exercise that is so important.  So gentle exercise that is comfortable and makes you feel good is likely to be repeated and done for longer.  If you go out and exercise too hard and get tired you will not exercise for so long and will probably come to a stop as your body tells you quite rightly you have done too much and to rest.  So easily does it.