27 CFR 70.221: Period of limitations upon assessment.

(a) The amount of any tax imposed by the Internal Revenue Code (other than a tax collected by means of stamps) shall be assessed within 3 years after the return was filed. For rules applicable in cases where the return is filed prior to the due date thereof, see section 6501(b) of the Internal Revenue Code. In the case of taxes payable by stamps, assessment shall be made at any time after the tax becomes due and before the expiration of 3 years after the date on which any part of the tax was paid. For exceptions and additional rules, see subsections (b) and (c) of section 6501 of the Internal Revenue Code.

(b) No proceeding in court without assessment for the collection of any tax shall be begun after the expiration of the applicable period for the assessment of such tax.