There are two categories of narrations in this regard which are apparently varied giving differing reports:
1.  The first category of narrations has, for the major part, been extracted from Sunni sources.[1] According to it, the period during which the Imam of Time, Imam Mahdi (AS), will rule the world has been reported differently. For instance, according to some reports, he will rule the world for twenty years while based on some other reports he will rule for thirty or forty years..
A) Huzaifa narrates from the Holy Prophet (S) that he said: "Al Mahdi is a man from my children. His face is like a glittering star, his skin is the color of an Arab, and his body is like that of Israel.  He will fill the earth with justice just as it will have been filled by injustice, and the dwellers of Heaven and Earth and even the birds will be pleased with his Caliphate. He will rule for twenty years."[2]
B) The Prophet (S) said: "Mahdi (AS) will rule for thirty or forty years."[3]
The second category of narrations has been cited from Shia sources according to which the duration of the government of Imam Mahdi (AS) has been reported to be seventy years.
A) Imam Sadiq (AS) said: The Riser (Qaim) of the Family of Muhammad (S) will rule for seven years. The days and nights during Mahdi”s time will be so lengthy that one year of that time will be like ten years of today and (seven) years of Mahdi will be equal to seventy years of your reckoning.[4]
B) Imam Baqir (AS) says in a lengthy tradition: "When Qaim Aal-e Muhammad shall rise up, he will go to Kufa… he will rule for seven years and each year of that time will be equal to ten years.[5]