When did God create Hell? Assuming a belief in a literal eternal hell believed by traditional Christian beliefs, when is hell believed to have been created by God?  A good answer will include scriptural support for when Hell was created.
My attempt of an answer based on the doctrine of the Catholic Church.

First, I could not find any explicit doctrine/dogma on this. Thus, my answer is inferred from other dogmatic definitions.


Hell and Genesis:

Let us start with the fact that, if God dwells in Heaven (e.g. John 14:2), Heaven has always existed (unless God "moved", which is odd). At some point, as related in Genesis, God created the universe and Earth. However, there is no direct mention of God creating a physical place named Hell. 
But Hell already existed at the time of Jesus resurrection:

Remember that Jesus descended into hell. Thus, hell must already exist. 

In effect, the Catholic Church teaches that there is a General (or Last) Judgment (which is to occur when Jesus comes again, and where bodies will be resurrected), and a Particular Judgment, which occurs immediate after the death of the body. In the latter, which has happened since Adam and Eve, the soul is sent to wherever it corresponds depending on the status of sin the individual is at.  However, before Jesus erased original sin with his death, even saints and good people were "sent to hell" (to be precise, this place is called limbus patris, "in which the souls of the just who died before Christ awaited their admission to heaven; for in the meantime heaven was closed against them in punishment for the sin of Adam;"). 


All this means that Hell must have existed as a destination of the soul since the death of Adam and Eve.  


Maybe hell is not a physical place, but just a state of separation from God.

This is in fact possible for now.  The Catholic Church has not decided anything on this. Many theologians and church fathers believed hell to be a physical place inside the Earth, in which case, God would have created hell just as he made Earth (for details, see "Names and Place of Hell" here). Yet, in my view it is only evident that, if after the General Judgment every human being is to have its body resurrected, Hell must by then be a physical place.  


In conclusion, hell, at least as a place for destination of disembodied souls, exists since the creation of mankind.  Whether it was "created" by God or not, it is open to debate, as this depends among other things on whether hell is a physical place or it is simply the "absence of God".  What is clear however is that in the General Judgment bodies will be resurrected, at which, given my limited human mind, it means hell as a physical place must definitively exist, and hence, be created by God.