Several years ago, Bicycling magazine published the following data:
Calories / lb-min
Add 22 calories/hour for each 100 ft of elevation
To translate this into a calorie burn rate, multiply the calories/lb-hr number by your weight in pounds and by the number of minutes you rode. For a 150 lb rider near sea level, cruising at 25 km/hour burns about 500 calories per hour. Riding at 35 km/hour burns about 900 calories per hour under the same conditions.
Your body's carbohydrate reserves amount to two or three thousand calories, and your fat reserves are more like 70,000 calories. For a typical rider, about 21% of your energy comes from fat, on average, and the rest from carbohydrates; with more carbohydrates being burned at higher levels of exertion. caffeine will increase the percentage of fat consumed for energy to 40% for the same exertion. When you run out of carbohydrates as an energy source, you "bonk" and have to slow right down unless you replace the energy as you use it.
This is just a rough estimate; variables such as the bike, terrain, hills, the aerodynamic efficiency of your tuck, and how efficient your body is will all impact the amount of calories burned quite substantially.