The bet was made shortly after Satan read Revelation, so early in the second century.
With respect to your previous comments, God knows everything, God will win.
Here's the logic I've tentatively settled on (unless someone blows this up on me):
Like everything in the Bible, the version of Revelation we have has to be interpreted correctly. So, when God shows it to us, common sense says that the correct interpretation of the future currently in the Bible is: Satan and sinning humans, change your ways or Revelation is what will happen to you. It's God's final warning.
As a result, Satan knows he must do something else, otherwise his butt will burn. Rather than stop sinning and return to worshipping and serving God, Satan cooks up the challenge, which, if he succeeds, offers him a different way to escape the fire while simultaneously forcing God and the angels to leave Earth, so Satan can rule it. Since God has a greater purpose for the challenge, he accepts.
The prophets, too, realize that Revelation is a warning (as are many Bible stories), not to be read literally (e.g., prove your obedience to God by killing your son, unless an angel stays your hand), so they have no problem writing it the way they did, which was done under the guidance of the Holy Spirit.
Humans, being smart too, have enough intelligence that they can figure this out as well (hell, I did, and my brain's not exactly firing on all thrusters, lol). It's just that most humans suck at interpreting the Bible correctly (45,000 denominations and counting), so most Christians think Revelation is literally what will happen.
Many Catholics have begun to realize what God intended, and have changed their interpretation, including the fact that much of the wording in Revelation is figurative, not literal. They too realize God is saying: Satan and sinning humans, change your ways, or Revelation is what will happen to you.
One might ask why God didn't simply come right out and say the above words. Sometimes he does, sometimes he doesn't. He didn't, for example, include an explicit statement prior to the story of the Flood (sinning humans, changes your ways, or I'll drown you all). Similarly, in Genesis, in the story of Adam & Eve, he didn't say, sinning humans, don't disobey me (i.e., don't eat the fruit), or I'll punish you severely, although he does say that elsewhere. Tons of examples where God has decided to show, not tell.
Here, too, dual timelines are no longer needed.