If you don't pay off your credit card balance in full at the end of the billing cycle (when you receive your statement), you will have to pay finance charges. How much you will ultimately end up paying will be based on your balance, the interest rate (determined by the issuing bank) and how long you finance (don’t pay off) the balance.
Your credit card statement includes a box that shows how long it will take you to pay off your balance if you make only the required minimum monthly payments, and also how much you would have to pay each month to pay your balance off in three years.
There are many online calculators that will allow you to enter your specific amounts (of debt and anticipated payments) and tell you exactly how long it will take for you to be debt-free. A couple of calculators to try are the ones offered by the Bankrate.com and Navy Federal Credit Union.