26 CFR § 301.6511(a)-1 - Period of limitation on filing claim.
(1) If a return is filed, a claim for credit or refund of an overpayment must be filed by the taxpayer within 3 years from the time the return was filed or within 2 years from the time the tax was paid, whichever of such periods expires the later.
(b) In the case of any tax payable by means of a stamp, a claim for credit or refund of an overpayment of such tax must be filed by the taxpayer within 3 years from the time the tax was paid. For provisions relating to redemption of unsued stamps, see section 6805.
(c) For limitations on allowance of credit or refund, special rules, and exceptions, see subsections (b) through (e) of section 6511. For limitations in the case of a petition to the Tax Court, see section 6512. For rules as to time return is deemed filed and tax considered paid, see section 6513.