In the long interview with Mexican broadcaster Televisa, released on the second anniversary of his surprise election, Francis also said he "did not mind" being pope but would like to be able to go out in Rome unrecognized for a pizza.
"I have the feeling that my pontificate will be brief - four or five years, even two or three. Two have already passed. It's a somewhat strange sensation," he said, according to a Vatican translation from Spanish.
"I feel that the Lord has placed me here for a short time," the Argentine-born pontiff said.
Francis, apparently in good health at 78, said "I share the idea of what Benedict did." In 2013, former
Pope Benedict became the first head of the Roman Catholic Church in 600 years to resign instead of ruling until he died.
"In general, I think what Benedict so courageously did was to open the door to the popes emeritus. Benedict should not be considered an exception, but an institution," Francis said.
However, he said he did not like the idea of an automatic retirement age for popes, such as at age 80.
In the 17-page interview, Francis also said the fact he is the first pope from Latin America compelled him to speak out on behalf of migrants and the poor because his ancestors had to move from Italy to Argentina to find work.
"People are being discarded and forced to seek employment elsewhere," said Francis, whose first trip after his election was to the Italian island of Lampedusa to pay tribute to thousands of migrants who have died trying to reach Europe.
Francis, who in the past has called for more regulation of markets, denounced "the injustice of wealth," saying it was a mortal sin to give someone an unjust salary or for the rich to take advantage of the poor.
On the lighter side, Francis said "I do not mind," when asked if he liked being pope.
"The only thing I would like is to go out one day, without being recognized, and go to a pizzeria for a pizza," he said, adding that he missed his days as a bishop in Buenos Aires, when he could move about the city freely.
"In Buenos Aires, I was a rover," he said.