I am agree with the answers above that after the 2 years.

But there are some news and its true too. you may come after 6 month if the following things are ok with you.
01. Its depend on your current profession (Eg: Accountant, Manager,Engineer)they will proceed visa even not completed 6 month.
02.shuold not be any police case or criminal records or Bills pending under your names.
03. Luck

Its not declared by qatar goverment, just I heard by some experience Mandoobs(PRO)