5 USC 3346 - Time limitation

(a) Except in the case of a vacancy caused by sickness, the person serving as an acting officer as described under section 3345 may serve in the office
(1) for no longer than 210 days beginning on the date the vacancy occurs; or
(2) 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.
(b) 

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(1) If the first nomination for the office is rejected by the Senate, withdrawn, or returned to the President by the Senate, the person may continue to serve as the acting officer for no more than 210 days after the date of such rejection,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.
(2) Notwithstanding paragraph (1), if a second nomination for the office is submitted to the Senate after the rejection, withdrawal, or return of the first nomination, the person serving as the acting officer may continue to serve
(A) until the second nomination is confirmed; or
(B) for no more than 210 days after the second nomination is rejected, withdrawn, or returned.
(c) 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.