Did some testing after you asked this. The tests went as followed:
Method: I went outside of New Tristram and used the Hex ability (unruned) against the zombies there. I unequipped and reequipped INT and Attack Speed items, as well as my weapon to test if there were any changes in duration or rate.
In a group of enemies the Fetish Shaman will:
- Hex 4 targets over his 12 seconds active
- Not Hex Targets already Hexed
- Hex targets while other targets are still Hexed (other targets
will remain Hexed for the full duration as well).
With a single target the Fetish Shaman will:
- Hex the 1 target 2 or 3 times over his 12 seconds active.
- Will only Hex the target if the target is no longer Hexed.
Running several tests with a Stopwatch on my Android I found that:
- The single target was Hexed for around 5.5 seconds.
- The Fetish Shaman will Hex a random target in a group of mobs around every
Additionally, changing my stats around (adding Attack Speed and INT, and changing my weapon) appeared to have no effect on how long the Fetish Shaman is active or the duration the target is Hexed.
Given these tests, I believe that there is no way to increase how long the Fetish Shaman is active and how often he turns an enemy into a chicken, and that there is no way to increase the duration of the chicken effect.
I'd also like to note that there can be a short delay between the Fetish Shaman spawning, and then using the polymorph ability. Which is why he may only Hex a single target 2 times instead of 3. 5.5s * 2 = 11, so if the Fetish Shaman takes a second to cast on spawn he'll miss the chance to polymorph the target a third time before he despawns.