In the Raleigh-Durham market, that would have been, on a permanent basis, between 1968 and 1971. Let me explain: When WRDU-TV 28 in Durham signed on in 1968, it was officially the market's NBC affiliate, though Durham's WTVD, channel 11, then the market's CBS affiliate, was also affiliated with NBC and had cherry-picked stronger programmingto work into its schedule since WRAL-TV, channel 5 in Raleigh, dropped NBC for ABC in 1962. The FCC had to get involved so that WTVD went with CBS fulltime and WRDU with NBC in 1971.
Delving even further back into our market's unique history, we actually had all three networks for a brief period from 1956-1959: The first station to sign on here, in 1953, was WNAO-TV, channel 28 in Raleigh, a CBS affiliate. In 1954, Durham's WTVD signed on as an NBC affiliate with a secondary ABC affiliation. In 1956, Raleigh's second station, WRAL-TV, signed on as an NBC affiliate, with CBS shifting to WTVD and ABC, to WNAO. With the major disadvantages of UHF at that time, especially in a vast market such as this one going up against two VHFs, WNAO went out of business in 1959, with WRAL and WTVD splitting ABC until 1962.
Except for operating on the same channel, the 1968 Durham-licensed WRDU-TV is in no way connected to the 1953 Raleigh-licensed WNAO-TV. NBC would stick with channel 28 as WRDU (1968-1978), WPTF-TV (1978-1991) and WRDC (1991-1995) before moving to a 1988 sign-on, Goldsboro-licensed WNCN, channel 17, which has an interesting history all its own (but perhaps one for another thread). As one can imagine with all the changes, NBC is traditionally not very strong in this market.