The subtitle duration is calculated automatically by multiplying each character of the predefined text times a configurable parameter. By default, this value is set to 0,08 seconds. A subtitle text with a length of 50 characters would therefore be displayed for 4 seconds.
To change the default value navigate to Blueprints/Core/Settings and open BP_SubtileDuration. Inside this structure blueprint simply change the default value for DisplayTimePerCharacter (note that “Character” here means literal characters).
The calculation happens in a global function you can find inside BP_StoryAdv_FunctionLibrary: