Network Goodson-Todman programs started carrying copyright dates in late 1977/early 1978. (One exception was evidently the ABC "Password"; the All-Stars finale has a 1975 copyright notice to Peak Productions.) Syndicated G-T shows always carried copyright dates AFAIK.
This is *only* speculation, but the adoption of copyright notices may have had something to do with the Copyright Act of 1976 coming into force on 1-1-1978. The previous legislation, the Copyright Act of 1909, was passed when broadcasting media were essentially non-existent, and until the new law came into force, the products of new technology had to have their copyrights interpreted through somewhat antiquated standards.
Some games did have copyright notices before the Goodson-Todman shows; I think the circulating 1976 episodes of "Wheel of Fortune" have them.
Daytime serials varied widely in terms of carrying copyright notices. "Dark Shadows" did from its beginning, and "Ryan's Hope" also had copyright notices prior to 1978, but the Proctor and Gamble dramas didn't have them until 1980 or so. I also haven't seen copyright notices on "The Doctors" episodes from 1967 to 1976 that have been aired on RetroTV.