14 CFR 61.39

(a) Except as provided in paragraphs (b) and (c) of this section, to be eligible for a practical test for a certificate or rating issued under this part, an applicant must:
(1) Pass the required knowledge test within the 24-calendar-month period preceding the month the applicant completes the practical test, if a knowledge test is required;


(b) Notwithstanding the provisions of paragraphs (a)(1) and (2) of this section, an applicant for an airline transport pilot certificate or an additional rating to an airline transport certificate may take the practical test for that certificate or rating with an expired knowledge test report, provided that the applicant:
(1) Is employed as a flight crewmember by a certificate holder under part 121, 125, or 135 of this chapter at the time of the practical test and has satisfactorily accomplished that operator's approved--
(i) Pilot in command aircraft qualification training program that is appropriate to the certificate and rating sought; and
(ii) Qualification training requirements appropriate to the certificate and rating sought; or


I stand corrected. The knowledge test DOES expire. However, one may take the practical test with an expired knowledge test provided they comply with 61.39 (b)(1) or (b)(2).