Jade Empire isn't as transparent with its mechanics as some games (unlike KotOR which tends to be quite transparent, released by the same company around the same time), so sometimes to get specifics of styles, I actually check costs, times, etc.

This of course includes Duration Increase.

In a game like this, most players (most devs, tbh) wouldn't take the time to test individual aspects of things like this, so I sometimes find that some things slip under the radar and actually don't work as advertised.

I've tested how long effects last for a few different styles (both Support & Magic) and I'm finding it quite strange that going from no points (0/5) to max Duration Increase (5/5) doesn't change the duration of effects AT ALL.

If there's a particular explanation for this, I haven't found it so far, but Duration Increase appears to be completely useless.

(I'm testing this on First Brother Kai of the Black Leopard School, who's useful for testing things since you can spar with him whenever.)

Ultimately, my question becomes: Is Duration Increase actually useless?

Good call, I saved before fighting some Mummies (who surprisingly don't appear to have immunity to any style) and timed numerous styles and you're right. The game seems to make some kind of distinction between certain enemies, though I'm not certain I'd be able to tell on sight who's resistant or not.

Against Mummies:
-Heavenly Wave's Power Attack lasts 5 seconds, +1 second per Duration Increase. (Against First Brother Kai, it lasts 5 seconds, regardless of Duration Increase.)
-Storm Dragon's AoE attack lasts 5 seconds, +1 second per Duration Increase. (Against First Brother Kai, it lasts 5 seconds, regardless of Duration Increase.)
-Paralyzing Palm's Power Attack lasts 10 seconds, +1 second per Duration Increase. (Against First Brother Kai, it lasts for all of 1 second, regardless of Duration Increase!)
-Ice Shard's Power Attack lasts 11 seconds, +1 second per Duration Increase. (Against First Brother Kai, it lasts 3 seconds, regardless of Duration Increase.)
-Stone Immortal's Power Attack lasts 11 seconds, +1 second per Duration Increase. (Against First Brother Kai, it lasts 5 seconds, regardless of Duration Increase.)
-Spirit Thief's Power Attack lasts 3 seconds, +0.1 second per Duration Increase, which is pathetic. (Against First Brother Kai, it lasts 1 second, regardless of Duration Increase.)

It's quite an odd design decision to me, not that some enemies are more resistant to the effects, but that Duration Increase makes no difference against those particular enemies.

Those are the only two enemies I've got detailed notes of particular times on, so not sure if it varies more between other enemies.

Ultimately, in answer to my original question, I'd say it definitely makes a difference (not counting Spirit Thief), but ONLY against regular enemies. It'd be useful in trying to lock down a few particular enemies when facing a crowd for a longer period.

EDIT: Oh, I should mention this was on Master difficulty (2/4), not sure if it's different on other difficulties.

Wow, well done test. I believe it will result the same for other difficulties. The only difference among them are enemies' hitpoint, damage and aggressiveness.

I think JE is good in taking care durations. The one that I consider as a flaw is the way the game does with resistances. There are only 2 possibilities, immune and normal. I tried to assign any numbers beside 0 and 100 at ressistance. 2da, but they re always work as it is 100 (immune).

I made my own 2da and did overwrite all on damage script to make it work like a creature has 75% resistance against some styles.