How long should I stay on bike if my heart rate is almost maxed for burning fat?

I ride an exercise bike, and I'm trying to burn fat. I heard 30 minutes you burn fat afterwards. However, if you've got the heart rate already up...way up before 30 minutes...like sprints but consistently, you should be burning fat way before 30 minutes on the count of High Intensity Cardio. Now how long does it usually take doing high intensity cardio to start burning fat when you exercise for the average body?

They usually say once you start to really sweat if when you are burning.

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