How many hours of cycling is equivalent 30 mins of running?

Is there any general formula for equating running with cycling, in terms of improvement/maintenance of aerobic endurance?

For instance, if I cycle (on a road) and try to maintain a constant heart rate of 170 bpm, for how long do I need to cycle for the workout to have the same effect as, say, 30 minutes of running at 170 bpm?
 
There is no real general formula for equating running with cycling but you travel roughly 4 times the distance on a bike for the same effort.
 
I guess 2hours. Cycling is not that hard than running,you can burn calories at a short time. Although as said,there is no equation on computing the equivalent effort in running and cycling.
 
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