A revised return can be filed for correcting ant omission or wrong syatement made in the revised return, because the revise return replaces the original return. For example: If return for A.Y.10-11 is filed on 31.7.10 and revised on 15.2.11 (before the assessment is completed) such revised return can be revised at any time upto 31.3.2012 or before the completion of assessment ., whichever is earlier.
So upto my knowledge can revise the return upto the last day of the next assessment year or before the assessment whichever is earlier.