VATICAN CITY - The conclave to elect a successor to the retired Pope Benedict XVI could take, hours, days or months. The only certainty is that it will continue until two-thirds of the voting cardinals agree.

The shortest conclaves in church history have chosen popes in a matter of hours.

But in 1268, a conclave began that lasted nearly three years. Frustrated Catholics eventually tore the roof off the building where the cardinals were staying and restricted their meals to bread and water to help them make up their minds.

More recently, the cardinals have, on average, chosen a pope in about three days.

The youngest pope, John XII, was 18 when he was elected more than 1,000 years ago.

Only one man selected pope was not a cardinal first. Archbishop Bartolomeo Prignano of Bari in Italy was named Pope Urban VI back in 1378.

AP wire services contributed to this report.