Gethseme on 3. May. 2017.
Rhykker, For Decay Golem's buff, note that when you cast the spell, the golem's shoulders and head start glowing bright yellow for the duration of the buff. My necro (just got into the beta so I'm leveling it while watching these vids :P) is only 29 so I don't have Decay golem unlocked yet, so I cannot test the buff duration, but just casting the ability on a corpse and watching how long it takes for the shoulders/head to stop glowing will tell you the duration of the buff.