As of 2023, the longest interest-free period is offered by Citi Diamond Preferred Card, issued by Citibank. The card had a 0% interest rate on balance transfers for 21 months. The card also offers a 0% interest rate on purchases for 12 months. After the introductory period ends, the interest rate will revert to the 16.74% – 26.74% range. The exact interest rate depends on one’s credit score and credit history