Emergency contraception (EC) should be used as soon as possible after unprotected sex or contraceptive failure, ideally within 72 hours (3 days) but may be effective up to 5 days after intercourse, depending on the type of EC used. The effectiveness of EC varies depending on factors such as timing, type of EC, and individual characteristics. Generally, EC reduces the risk of pregnancy by about 75-89% when taken within 72 hours after unprotected sex.