5 U.S.C. § 3346: Time limitation

for no longer than 210 days beginning on the date the vacancy occurs; or subject to subsection (b), once a first or second nomination for the office is submitted to the Senate, from the date of such nomination for the period that the nomination is pending in the Senate. If the first nomination for the office is rejected by the Senate, withdrawn, or returned to the President by the Senate, the person may continue to serve as the acting officer for no more than 210 days after the date of such rejection, withdrawal, or return. until the second nomination is confirmed; or for no more than 210 days after the second nomination is rejected, withdrawn, or returned. If a vacancy occurs during an adjournment of the Congress sine die, the 210-day period under subsection (a) shall begin on the date that the Senate first reconvenes.