It really all depends to your weight and your workout intensity... Are you riding outdoors? Spin class? or a bike at the gym?
Typically in a spin class (depending to your weight and intensity of your workout) you can burn 400 - 1000 cals... so I guess you can assume that you will be buring 100 - 250 cals in 15 min. But again there are a lot of factores... if you give more details as where and what conditions you are riding I or someone else can give you a better answer.
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