The Code of Canon Law calls for the heads of every diocese in the world to make their formal visit ad limina apostolorum ("to the threshold of the apostles") every five years, but there are now almost 2,900 dioceses in the world and the 85-year-old pope also has other obligations as well.
Archbishop Lorenzo Baldisseri, secretary of the Congregation for Bishops, which coordinates the visits, told Catholic News Service on Monday, "the firm principle is that the pope must meet the bishops of the whole world regularly."
The five-year rhythm set by canon law provides concrete guidance but is not always possible to follow because of the number of bishops in the world, the pope's schedule and the schedules of the bishops.
In an interview published by the Vatican newspaper Saturday, Baldisseri said that when the bishops of France finished their ad limina visits in November, members of every bishops' conference in the world had met Pope Benedict.
The order in which bishops' conferences make the visits is not strictly set, which means that though the French bishops had never had an ad limina with Pope Benedict, the bishops of Papua New Guinea had two: one in June 2005 and the second in June 2012.
Government restrictions prevent the bishops of communist-controlled mainland China from making their ad limina visits, although the bishops of Hong Kong and Macau had their meetings with Pope Benedict in 2008.
Baldisseri told L'Osservatore Romano, the Vatican newspaper, that the importance of visiting and consulting with the pope, the successor of St. Peter, goes back to St. Paul's description in the Letter to the Galatians of returning to Jerusalem for consultations with St. Peter.
However, the archbishop said, it wasn't until 743 that Pope Zachary made it a universal rule.
The rule was reconfirmed by Pope Sixtus V in 1585.
"The bishops are invited periodically to come to Rome to see Peter, make a pilgrimage to the tombs of the apostles Peter and Paul -- founders of the church of Rome -- and to express and reinforce the unity and collegiality of the church," he said.
The visits are not just "a simple juridical-administrative exercise," he said, but "an experience of pastoral communion, participating in the concerns and hopes" of the church on the local and universal levels.
The relationship between the bishops and the pope, he said, "cannot simply be sporadic or spontaneous, but must be regular and ordered because we are dealing with the life of the church in its universal and particular dimensions."
The Italian bishops made their first and only ad limina visits with Pope Benedict from November 2006 to April 2007, which means their second visits are coming six or seven years later.
Bishops from Mexico, Austria, Poland and other countries that had ad limina visits in 2005, the first year of Pope Benedict's pontificate, will have to wait until 2014 or beyond.
The special Year of Faith calendars of Pope Benedict and of local bishops, together with the size of the Italian bishops' conference, "will not permit the visits of other episcopal conferences" this year, Baldisseri said.