Editing Arcanum isn't easy, there's a guide to how to edit a lot of things here:
GameFAQs: Arcanum: Of Steamworks and Magick Obscura (PC) Background/Schematic Editing FAQ by Count Tyrant
However whilst I've done a lot of editing of Arcanum, I don't ever recall being able to change the duration of effects.
This site
Dimensions of Arcanum - Tinkerer's Workshop has a lot of helpful files, maybe you can find what you're looking for there.
Best of luck, and remember to back up everything before editing anything.