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The longest that's undisputedly a single story is "The Daleks' Master Plan". It was twelve episodes of 25 minutes apiece, so about 300 minutes (5 hours) in total. Unfortunately, nine of those episodes are missing from the archive.

The longest that was broadcast under a single title was "The Trial of a Time Lord". It consisted of fourteen episodes, of which thirteen were of 25 minutes & one (the finalé) was of 30 minutes, a total of 355 minutes (5 hours 55 minutes). However, "The Trial..." is usually considered to be four linked stories: "The Mysterious Planet" (episodes 1 - 4), "Mindwarp" (episodes 5 - 8), "Terror of the Vervoids" (episodes 9 - 12) & "The Ultimate Foe" (episodes 13 & 14).

In terms of the franchise, the longest single story is "Miracle Day" (Torchwood story), the Torchwood story arc that ran for 10 hour-long episodes, or about the equivalent of 20 old-style Doctor Who episodes.