If an original return was filed within the extension period, an amended return for a claim for refund must be filed within:

One year from the date of overpayment.

Two years from the date the original tax return was filed or within one year from the date of overpayment, whichever period expires later.

Three years from the date the original tax return was filed or within one year from the date of overpayment, whichever period expires later.

Four years from the original due date (April 15th) or within one year from the date of overpayment, whichever period expires later.

that's all the info