It depends on how much you weigh now. The more you weigh, the more calories it takes to maintain your system. A person that is grossly overweight can lose 20 pounds quickly but it takes longer to lose the 20 pounds if you are only 20 pounds overweight. Also, it's not just exercise, how much you eat and what you eat play an important part. For example, if the additional exercise makes you hungry and you increase the number of calories you eat to the number you're burning in the additional exercise, you won't lose any.
Bottom line, you have to burn 3500 calories more than you eat to lose one pound. If you eat 3500 more than you burn, you gain one pound. The rate at which you lose will depend on the difference. It is fairly easy to look up the calories in different foods but very difficult to calculate how many you are burning.