You must file that return within 3 years of the date it was originally due, including an extension if you filed one. This is the statute of limitations for refunds. But there is a refinement of that rule that prohibits refunds of payments made more than 2 years before the claim for refund is made once the 3-year period has expired.
Observation: The application of these rules can be quite complex and sometimes baffling - even for tax professionals.