It hits like a soulfire
It has a 2 second cast time
It has a 1 second flight time
Immolate has a 15 second base duration
Immolate will drop to 12-13 seconds with haste before adding a new tick
Combined with the 12 second cd of HoG, 2 second base cast time, 1 second flight time, refreshing immolate with a base 15 second duration alone is hard, haste effects can make it harder or easier depending on your gear level
There are haste levels where its impossible to refresh Immolate with HoG
What I just figured out today was that depending on "when" you refresh with HoG, like say 0.2 seconds before an immolate tick, sometimes immolate won't refresh to its proper full duration.
Today I was casting immolate and refreshing with HoG for a good 10 minutes, letting it fall off every 2 minutes so I could restart. What I noticed is sometimes immolate would refresh to the full 15 seconds, and sometimes it would refresh to roughly 13 seconds. With a 13 second duration on immolate and my 1.92 second cast time with a 12 second cd and a 1 second flight time, its impossible to refresh.
Something needs to be done about Immolates duration for Demonology, and Fel Flame isn't the answer.