Longest running 'non-animated' character on prime time television (US)

Two characters played by the same actors dominated the prime time slots on American TV for 20 years each. The first was Marshall Matt Dillon, portrayed by James Arness (USA) on the western Gunsmoke (CBS, USA, 1955-1975); the second was Dr Frasier Crane (aka Kelsey Grammer, USA) in Cheers (NBC, USA, 1982-1993; appeared 1984-993), Wings (NBC, USA, 1990-1997; a guest appearance in 1992) and Frasier (NBC, USA, 1993-2004). The original radio version of Gunsmoke ran from 1952 to 1961, so Dillon has been a longer-running character; however, he has been portrayed by more than one actor.