Continuity mistake: This is a mistake within a mistake: Niles and Frazier are trying to get the wrapped presents out of F.'s apartment and replace them with others, to play a trick on their dad. They are getting completely mixed up, and presents are being taken out and then returned by mistake, mostly by Frasier. At one point Niles has a pile of gifts which he puts down on the table behind the couch - the shot shows him holding them, but the next shot shows a completely different pile of presents, even though it is supposed to be the same pile. Frasier picks this pile up. It soon changes again in the next shots.
Answer: It equals the record of James Arness of "Gunsmoke" for the actor who's played the same part on TV NOT in a soap opera. If you include soaps there are two main contenders - William Roache, who's been playing Ken Barlow in "Coronation Street" in the UK since 1960. Don Hastings has been playing Robert Hughes in US soap "As the World Turns" since 1960 as well.