After the reappearance of Imam Mahdi, how long will it take for him to establish the rule of justice?

Based on the narrations that are available to us, he will reappear in Mecca. After the elite of his followers join him there, they will then advance toward 'Iraq, Shām (the present day, Syria), and from there toward Jerusalem. Thereafter the lands of Europe, Turkey, China, Afghanistan, and of other regions will be conquered by him, and so after eight consecutive months of struggling, the rule of justice will be established, and peace and security will become prevalent. But about the duration of his rule, there is inconsistency amongst the narrations. Some of the related durations are as follows: 19 years and some months; seven years; 40 years; 10 years and the like.