As it turns out, the Skill Duration improvements in Arche Tuning only improve your existing bonus, ie, if you dont have Skill Duration buffs via Modules, Reactors, and External Components, the Tuning nodes will give you next to nothing. This has been noticed by a player who goes by the name of Ezio on Discord.
Source: N4G PC The First Descendant Players Deem the Skill Duration Arche Tuning Nodes Next to Useless