In total there are 3 sources from which one can get the Increased Abyssal Decay duration affix: Helmets, Body Armour and Sentinel Grand Idols. But only the Helmet and Body Armour affixes have an effect on the Abyssal Decay DoT from Abyssal Echoes.
I did multiple tests on target dummies, recording the dot duration (± 0.10s human error) and the on-hit effect damage of abyssal decay immediately after casting abyssal echoes and came up with these results:
| Total Increased duration | Abyssal Decay duration | On-hit effect Damage | |
|---|---|---|---|
| Base | +0% | 6s | 9712 |
| Helmet | +43% | ~8.504s | 13888 |
| Helmet + Body Armour | +108% | ~12.783s | 20201 |
| Helmet + Body Armour + 4 Grand Idols | +167% | ~12.783s | 20201 |
Here’s a short comparison video showing the duration of Abyssal Decay and how the idols have no effect.
Expected Behaviour
The Increased Abyssal Decay duration affix for Sentinel Grand Idols should behave exactly the same as the equally named affixes for Helmets and Body Armour.
Actual Behaviour
The Increased Abyssal Decay duration affix on Sentinel Grand Idols has no effect on the Abyssal Decay DoT.