Well again, no one can tell you for sure. In *general* .. if you are walking at a brisk pace (say 3.5 miles per hour or faster), you can estimate you'll burn 130 calories a mile or so. But again, that doesn't take into account your heart rate or anything like that.
For most people (and again this is a very general estimate), moderate intensity cardio (like brisk walking) will burn anywhere from 300 to 500 calories per hour. And again, that's very general and doesn't take into account your heart rate or general level of fitness and so forth.
Any exercise you do that gets your heart rate elevated for 20-30 minutes is going to help you lose weight, but walking and exercise alone won't do it. The most important part of losing fat is what you eat. You can walk until you fall over, but if you're eating non-nutritious food or too much food, you won't lose fat.
So you need to make sure that you balance proper eating with your walking.