47 C.F.R. § 74.34
Period of construction; certification of completion of construction
(a) Each aural and television broadcast auxiliary station authorized under subparts E and F of this part must be in operation within 18 months from the initial date of grant.
(b) Each remote pickup broadcast auxiliary station authorized under subpart D of this part must be in operation within 12 months from the initial date of grant.
(c)-(e) [Reserved]