The longest individual episode, in on-air running time, would be "The Five Doctors", which runs for 90 minutes or 100 minutes for the extended version.

Although each individual story had a separate title, the 23rd season consisted of one story arc in 14 episodes under the umbrella title "The Trial of a Time Lord" and season 16 was one story arc ("The Key to Time") with 6 stories/titles spanning 26 individual episodes. The next longest (under a single title) was "The Daleks' Master Plan" (12 episodes) spanning from Nov. 13, 1965 through Jan. 29, 1966.

Also in "The Last of the Time Lords"/"The Sound of Drums" is set over the space of 1 year making it the episode with the longest amount of fictional time