Ever heard someone say: “For fat loss, its best to job first thing in the morning!”. Well yes if you are looking to burn fat and there’s a reason why.
When you do cardio, your body burns stored energy.  Glycogen that is stored in your muscles is its primary source of fuel/energy. Fat is ..well..your energy reserves. Glycogen is the first thing your body will look for before looking elsewhere (fat) and glycogen levels rise/fall depending on your carb consumption.
So its “best” in the morning because your glycogen level are low right after you wake up (unless you eat lah). Starting your cardio or running session then and your body is forced to burn fat. But most of us would dread getting up that early, any day of the week! So here’s what you can do to make sure you are burning fat instead of glycogen. Avoid consuming carbs 3-4 hours prior to your run to “deplete” glycogen levels. So say you want to hit the road or treadmill @ 6pm, your last carb meal should be around lunch!
This is good for fat burning but not ideal for performance athletes who need quick energy from glycogen. That’s why football player like Maradona are not super lean (although they train daily) as they burn glycogen and not fat. They train for performance, not fat loss.
What about duration? 
For at least 20 minutes! Your body only has about a couple pounds of glycogen stored in your muscles. For the average Joe, its takes about 20 minutes to use up all glycogen before turning to fat. Hence, you should maintain your cardio session for at least 20 minutes.
Cardio FITT Guidelines
F Frequency Try and perform cardio 2-3 times per week to continually stimulate your heart and lungs.
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I Intensity Make sure you that keep the intensity up. Keep pushing for greater heights. Once in a while “a stroll in the park” provides good balance.
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T Time Minimum of 20 minutes but allow your body (and joints!) to get proper rest.
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T Type Diversity is key! Don’t perform the same run/track, day in and day out. Mix it up.