The admission criteria varies IIT to IIT and they all have their own set of role for admissions. And it also depends when they are releasing the application form so tentatively within a month or two right after the application forms come they conduct interview.

So below I am providing you the article which you can go through to check the entire admission criteria for all IITs -

http://www.engineering.careers360.com/articles/iit-mtech-admissions

Good Luck!!!