Reg. § 301.6501(a)-1 Period of limitations upon assessment and collection.
(a) The amount of any tax imposed by the 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 . In the case of taxes payable by stamp, assessment shall be made at any time after the tax became 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) to (g) of section , and for cross references to other provisions relating to limitations on assessment and collection, see sections and .
(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.