Check out this debt calculator from CNNMoney: http://money.cnn.com/calculator/pf/debt-free/
For each month, we calculate and add the interest accrued during that month to the amount you owed during the previous month. Then we subtract your monthly payment to arrive at the new amount owed. We repeat the process and track the number of months needed for the amount owed to reach $0. If you have multiple debt types, your debt-free date is based on the debt that will take the longest time to pay off.