Long Island Bishop William Murphy hits the mandatory church retirement age of 75 Thursday, setting the stage for a change in leadership in one of the nation's largest Roman Catholic dioceses.

Murphy must submit a letter of resignation to the Vatican on his 75th birthday.

The resignation could be accepted immediately and a replacement named, or he could continue leading the Diocese of Rockville Centre for weeks, months or even years, experts said.

Pope Francis has let "some people . . . stay in office well beyond the 75-year mark, and some, he's accepted their resignation right away. It's kind of unpredictable," said John Thavis, a longtime Vatican reporter and author of "The Vatican Diaries."

Murphy has said he's open to either possibility.

"I am perfectly content to do what he tells me," the 13-year bishop told Newsday in a February interview. "I'm very happy to keep doing the work. But, literally, I have prayed a lot about this."

"I try to have the notion of 'apatia.' I have to be indifferent. That doesn't mean I become passive. . . . I hope in the best sense, that if he wants me to go, that's God's will. If he wants me to stay, that's God's will."

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A number of factors determine how quickly a bishop is replaced, Thavis said.

"A lot depends on the health of the bishop and to some degree how he is perceived by his flock," he said. "In other words, if there are big problems, when they hit 75, quite often, OK, it's time to go."

Murphy indicated he has no major health problems. "If I were sick, I think I'd owe it to the diocese to tell them," he said.

The replacement process is highly secretive and not subject to hard-and-fast rules, experts said.

Pope Francis will ultimately select the new bishop and decide when he starts. Another key figure in the process is the pope's representative in the United States, the papal nuncio, Archbishop Carlo M. Vigano.

After gathering information from the local diocese and other sources, Vigano will submit three nominees to the Vatican's all-important Congregation of Bishops.

If the congregation approves the list -- which has a first choice, a second and a third -- it is forwarded to the pope. The congregation or the pope can reject the nominees and direct the nuncio to start over.

"We have seen that Pope Francis has increasingly been choosing his kind of bishop, which is a person who is on his wavelength when it comes to redirecting the church's energy toward what he calls the fringes or margins of society," Thavis said. "In short, a more pastoral approach."

Murphy was installed as bishop of the diocese, home to 1.5 million Catholics, on Sept. 5, 2001. He is the fourth bishop of the diocese since its founding in 1957.