27 CFR § 70.251 - Periods of limitation on suits by taxpayers.
(a) No suit or proceeding under section 7422(a) of the Internal Revenue Code for the recovery of any internal revenue tax, penalty, or other sum shall be begun until whichever of the following first occurs:
(1) The expiration of 6 months from the date of the filing of the claim for credit or refund, or
(2) A decision is rendered on such claim prior to the expiration of 6 months after the filing thereof. Except as provided in paragraph (b) of this section, no suit or proceeding for the recovery of any tax, penalty, or other sum imposed under the provision of 26 U.S.C. enforced and administered by the Bureau may be brought after the expiration of 2 years from the date of mailing, by either registered or certified mail, by an appropriate TTB officer, to a taxpayer of a statutory notice of disallowance of the part of the claim to which the suit or proceeding relates.
(b) The 2–year period described in paragraph (a) of this section may be extended if an agreement to extend the running of the period of limitations is executed. The agreement must be signed by the taxpayer or by an attorney, agent, trustee, or other fiduciary on behalf of the taxpayer. If the agreement is signed by a person other than the taxpayer, it shall be accompanied by an authenticated copy of the power of attorney or other legal evidence of the authority of such person to act on behalf of the taxpayer. If the taxpayer is a corporation, the agreement should be signed with the corporate name followed by the signature of a duly authorized officer of the corporation. The agreement will not be effective until signed by an appropriate officer.
(c) The taxpayer may sign a waiver of the requirement that the taxpayer be mailed a notice of disallowance. Such waiver is irrevocable and will commence the running of the 2–year period described in paragraph (a) of this section on the date the waiver is filed. The waiver shall set forth:
(1) The type of tax and the taxable period covered by the taxpayer's claim for refund;
(2) The amount of the claim;
(3) The amount of the claim disallowed;
(4) A statement that the taxpayer agrees the filing of the waiver will commence the running of the 2–year period provided for in section 6532(a)(1) as if a notice of disallowance had been sent the taxpayer by either registered or certified mail.
(d) Any consideration, reconsideration, or other action with respect to a claim after the mailing, by either registered or certified mail, of a notice of disallowance or after the execution of a waiver referred to in paragraph (c) of this section, shall not extend the period for bringing suit or other proceeding under section 7422(a) of the Internal Revenue Code.