Within 4 years from end of relevant AY, if AO comes to know that there is mistake
Within 6 monts from end of month in which assessee has made application for rectification.
So,if assessee comes to know that there is mistake in return which is not debatable then he can make application to AO on his own and AO have to rectify it within 6 months. But Assessee can make application within 4 years fron end of relevant AY.
For Ex. for AY 2009-10, timelimit is 31-3-2014. i.e.4 years from 31-3-2010.