When Was the Fire of God's Wrath Kindled?

The answer to this question is the answer to another question: How old is the said wrath of God?

That God speaks of this fire in connection to the sin of Israel does not mean that their sin was the reason for its kindling. In fact, the opposite is the case.

The fire had already been kindled. It is because of this that God speaks of their end.

There is A Fire of Torment In Sheol/Hades

God says that the fire kindled by His wrath burns even in the lowest of Sheol/Hades. This means that there is a place of torment in Sheol/Hades

This agrees with the torment in the fire in hades of the rich man [Luk 16:23-24] (in the story of poor Lazarus and the rich man told by the Lord Jesus).

The Abyss & the Fire of Torment In Sheol/Hades

We should note that Moses defines the part of Sheol/Hades where this fires of God's wrath burns as the "lowest sheol".

This corresponds to the Abyss which means bottomless [Rev 20:1,3] and confirms the fear of the demons in the demoniac,

“What do you want with us, Son of God?” they shouted. “Have you come here to torture us before the appointed time?”
And they begged him repeatedly not to order them to go into the Abyss. [Luk 8:31 cf., Mat 8:29]