How often do I have to run to burn fat?

Well obviously from the question I'm just trying to burn those few couple pounds of fat away from my stomach to burn fat and have more definition on my abs so how often do I have to run on either the elliptical or treadmill I have in my basement? and how long each time I run as well? And any other tips you have would be greatly appreciated!
How often do I have to run to burn fat?
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