In general, cycling is not as efficient at burning calories as running. Running 30 minutes usually gives you the same workout as cycling for an hour or more. It's easier on the knees, but the lack resistance makes it harder to push yourself. The upshot of all this is that you have to ride long distances to achieve the same workout. Usually a good workout for road cyclists is 25-30 or more miles averaging about 20 miles an hour. A good workout for mountain bikes is about 17-25 miles over three hours on a relatively flat trail, 6 miles or less if the trail involves a huge climb. I've heard from some other mtn bikers that you can easily burn 1500 calories an hour or more climbing.