Answered by Father Jeremiah Payne, St. Joseph Catholic Parish in Palm Bay, May 19, 2017.
Asked by Colin, an 8th grade student at Sacred Heart Catholic School, New Smyrna.
The short answer is that there is no specific amount of time required.
In earliest times, popes, like other bishops, were elected by the local clergy (priests) and laity (people) of the place where they would serve. This was also the case in Rome. The clergy of the Diocese of Rome would elect their bishop who, as the Successor of St. Peter, was also the Pope.
As history developed, the choosing of the Bishop of Rome was limited to the clergy, and then only certain clergy. These select clergy became known as the “Cardinal” clergy of the Diocese of Rome. So, the Cardinals would elect their bishop, the Pope.
As time passed still, the Cardinals also became an advisory body to the pope, a “college” or “senate”, and the desire to widen the influence led to making some clergy, not of the Diocese of Rome, “titular” Cardinals. This is mostly the case today that Cardinals are from all around the world, but serve also as clergy of the Diocese of Rome and are assigned a “titular” parish within the Eternal City as their parish.
While they advise the Pope on a good many things, their most solemn function is to elect the Bishop of Rome when the See of Peter becomes vacant because of resignation or death. While it is true that almost always the Bishop of Rome is selected from among the Cardinals, being a Cardinal is not an absolute requirement for being elected Pope.
To be elected Pope, one need only be qualified to be a Bishop; this means that he has to be a validly baptized male in full communion with the Catholic Church and, if not already, ordained to the diaconate and priesthood. That being said, there has not been a non-Cardinal elected to the See of Peter since 1378, Pope Urban VI, and that ended in the great Western Schism!