Fat burning percent is relative to exercise intensity.
As far as cycling is concerned, you will generally burn between 200-300 calories per hour of fat depending on your exercise intensity.
I found this whitepaper a few months ago that gives a good idea of how much fat is burned per hour.http://www.uni.edu/dolgener/Advanced_Sp ... dation.pdf
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Cliff notes :
Maximal fat oxidation occurs at about 60-65% of vo2max, and is about a maximum of 0.55g per min, which is 297 calories per hour (Note this is about 73% of max heartrate).
At 50% or 70% of vo2max, fat oxidation is about 0.45g per min, or 243 calories per hour.
At 40% or 80% of vo2max, fat oxidation is probably about 0.3g per min, or 162 calories per hour. Note, this wasn't tested in this whitepaper, and it's just a guess.
But really, the higher exercise intensities (70%+) also burn alot of glycogen, which relatively means that your burning more energy regardless, even though it's not as much fat as a lower intensity.
The paper also suggests that some riders can ride up to 85% of vo2max with no decrease in fat oxidation, i.e. still around 0.55g per min, or 300 cals an hour.