The CFPB says a debt does not generally expire or disappear until it is paid, but in many states a statute of limitations limits how long creditors and collectors can use legal action. Most states set limits of three to six years, some longer, and the period can vary by type of debt, your state and the state law named in your agreement. Some debts, such as federal student loans, have no statute of limitations.
When the period starts differs. In some states it begins when a required payment is missed; in others it counts from the most recent payment, even one made during collection. The CFPB warns that a partial payment, or acknowledging you owe an old debt even after the period ends, may restart it.
In most states collectors can still try to collect after the period expires, by letters or calls, if they follow the law. They cannot sue or threaten to sue. A suit on a time-barred debt violates the Fair Debt Collection Practices Act, but a court may still enter a judgment if you do not show up and raise the statute of limitations as a defense. The CFPB suggests consulting a lawyer to calculate the period for your debt.