The window for amending tax returns is 3 years from the date the tax return was originally filed.
"But once you’ve filed your return, you can’t be prosecuted for failing to file an amended return, even though something may happen after you file that makes it clear your original return contains mistakes. So first ask yourself whether the return you filed was accurate to your best knowledge when you filed it. If it was, you are probably safe in not filing an amendment."
CIRCULAR 230 NOTICE:
To the extent that this message contains advice regarding tax matters, it is not intended or written to be used, and it cannot be used, by any taxpayer for the purposes of avoiding penalties that may be imposed under the Internal Revenue Code or applicable state or local tax law provisions. This disclaimer is provided pursuant to U.S. Treasury Regulations governing tax practice.