Doing an early morning cardio session can help you start the day the right way! Doing low intensity cardio on an empty stomach is more likely to help your body use stored bodyfat as fuel for your workout rather than lean muscle.
As long as you keep the intensity low (in your fat burning zone, rather than working in your cardio zone) you should start to see significant benefits of this type of cardio after about 2 weeks.
Of course if you eat lots of unhealthy foods, then NO amount of cardio is going to help! You have to remember that the body continually adjusts its use of fat and carbohydrate for fuel depending on a variety of factors. Many researchers and professionals also claim it is counterproductive to workout on an empty stomach and some people note that they feel nauseous and tired during workouts. Your body might react slightly differently to the next person.
All I can say is if you’re keen, give it a try! Do your cardio on an empty stomach for 4 to 6 weeks and note the results. If it works for you, find a way to fit morning cardio on an empty stomach into your lifestyle. It’s how I stay lean with minimal effort all year round.