Dear Mary: Is there a law that says how long the grace period must be? (And if you don’t mind, exactly what is a grace period?) —Justin Dear Justin: In the world of lending and borrowing, the “grace period” is the number of days between the time you make a credit card purchase and when you will begin to pay interest on that short-term loan. Here’s how it works: When you pay your credit card balance in full during the “grace period,” you don’t have to pay any interest—the operative words here being “in full.” That means you pay the entire balance owing every month. If you don’t pay the entire balance and instead roll it over to the next month, you forfeit the grace period until your balance returns to $0. If you don’t pay that credit card account balance right down to $0 every month before the due …