Game concept
Scheme Phase Length
The game also links it as Phase Length.
From the game's encyclopedia
Phase Length refers to how many days it takes to complete a single Scheme Phase. Speed is affected by your Skill, Agents, and your Spymaster, as well as by the 'Support Schemes' Councillor Task if the Scheme is Hostile.
A lower Phase Length is beneficial because it will cause a Scheme to progress faster.
The game's text for patch 1.20.1.2. Words in blue are entries of the encyclopedia: hover one to read it, as in the game.
Scheme Phase Length is an entry of Crusader Kings III's in-game encyclopedia. The game explains it: "Phase Length refers to how many days it takes to complete a single Scheme Phase." It is part of Scheme. The game also links it as Phase Length. Patch 1.20.1.2, from game/common/game_concepts/00_game_concepts.txt and game/localization/english/game_concepts_l_english.yml.
Source: game/common/game_concepts/00_game_concepts.txt, game/localization/english/game_concepts_l_english.yml and the English localisation, Crusader Kings III 1.20.1.2.