Distance is somewhat irrelevant in bike riding. Time is a much better measurement, and is what most all riders (professional and amateur) use to measure their training.
That said, your level of fitness also will determine how much fat you burn. Someone that is in good physical condition will burn more fat per hour than someone who doesn't exercise very often.
1 pound of body fat contains 3500 calories. That means if you were to lose 1 pound of fat over the course of 1 week WITHOUT exercise, you would have to reduce your calorie intake by 500 calories each day. The average person eats around 1500-2000 calories a day, so 500 calories is a significant amount. The other option is to BURN an extra 500 calories each day through exercise. Again depending on your body type and fitness level, it will take between 1-2 hours of exercise to burn 500 calories. Reducing or even eliminating junk foods (sweets, soda pop, chips, etc.) will help speed up the process as they are usually high in calories and contain little nutrition anyway.