Sixty days after the date on which it was mailed (30 days in the case of a proposed assessment of withholding taxes, or, in the case of other taxes imposed by Chapter 11 of this title, 120 days if the taxpayer is outside the United States), a notice of proposed assessment under § 521(c) of this title shall constitute a final assessment of the amount of tax, interest, penalties, additional amounts and additions to the tax specified in such notice, excepting only those amounts as to which the taxpayer has filed a timely protest with the Director under § 523 of this title.