If taxes are owed then an amended tax return can be filed at any time.
If for a tax refund, then you have up to three years from the original due date of the tax return or if an extension was filed the extension date. For tax year 2015 the amended return has to be filed for a refund on or before April 15, 2019 or if a 2015 extension was filed, October 15, 2019.
