In an interview to mark the second anniversary of his election he said: "I have the feeling that my pontificate will be brief. Four or five years; I do not know, even two or three".
The Holy Father has hinted in the past that he could retire and again in this interview stressed that Pope Benedict's decision to step down in 2013 is not an exception.
He said his predecessor's resignation was "courageous".
"Benedict should not be considered an exception, but an institution. Maybe he will be the only one for a long time, maybe he will not be the only one.
"But an institutional door has been opened," he told Mexican television.
But the Pope rejected the suggestion an age limit should be set on the papacy.
"To say that one is in charge up to 80 years, creates a sensation that the pontificate is at its end and that would not be a good thing," he stressed.
When asked how long he would be at the Vatican he replied: "I do not know. But I feel that the Lord has placed me here for a short time, and nothing more.
"But it is a feeling. I always leave the possibility open."