Time and spark seem to follow the same curve, the reductions are on a curve. Spark is a little more flexible though since there are both skills and buffs that can improve that above the curve.
The max amount of reduction you can see in the GUI comes with the 16th coil past the number of coils required for the craft. Over 14 it’s laughable though.
I thought I had posted a breakdown in my old serpensarindustries threads but it must be buried around here. If I can find a breakdown soon I’ll link it. I’m confident enough to state 16 coils from memory.
For anyone reading who isn’t clear what I mean by “past the number of coils required for the craft” it’s just that the reductions don’t start until you’ve met the requirement, which means if something requires 600 power, you’re going to get reduction up to 18 coils. If the craft requires 1200 power, you’re going to get reductions up to 20 coils (4 required, 16 reducing) etc…
All the way up to some items (I think just some max coils) that actually require 24 advanced coils to craft. Once you hit 2400 in the power requirement, you will be able to see time/spark reductions out to the 24th coil. If the craft requires more than 2400 power you will never max the curve.
EDIT:
This thread is loaded:
Shows my breakdown of spark on a 0 power craft (the difference between 15 and 16 coils on that while fully buffed was one spark 
), time reductions on some high power crafts, and links to even more/older data.