1. This week?
Why it might be: He just got beat down by the Packers. More importantly, Childress's job security was the best weapon Favre had. There were endless debates on Sunday-morning-type shows about whether Childress should bench Favre; this was a stupid question because whether he should have, he was never going to. His job was in danger, and the only chance he had to keep it was if Favre turned the team around. Tarvaris can't do it, and even if he could, Childress had pushed in all his chips on Favre. Interim coach Leslie Frazier doesn't have any chips in on Favre, and if Brett keeps playing as badly as he has, Frazier would be more able to bench him for performance.
Why it might not be: Favre got beat-down score-wise by the Packers, but not physically beaten as much as he has so far this season. Furthermore, it was no secret that he didn't like Childress, so he'll probably want to give it another shot with Leslie Frazier at the helm--especially considered how this season's other interim coach, Jason Garret, has apparently worked a miracle with the Cowboys.
Odds: 1 in 10
2. Next week?
Why it might be: Again, Leslie Frazier may well be willing to bench Favre if he plays badly (which he has been doing quite a bit this season). Favre would be livid if he were pulled from the game, and would probably refuse to play for Frazier ever again.
Why it might not be: They're playing the Redskins, a team bad enough that they might make the Vikings look good.
Odds: 1 in 7
3. Early December?
Why it might be: If the Vikings keep losing, and if the Bears and Packers and New Orleans and/or Tampa Bay keep winning, the Vikings could be mathematically eliminated from the playoffs as early as the night of December 5th. With the opportunity to go to the Super Bowl finally, officially over, Favre would have no reason to keep playing.
Why it might not be: The Vikings might win a few games with Frazier (they do get to play Buffalo right after Washington), and the Bears and Packers have tough ones coming up. In all probability, the Vikings will technically stay alive a little longer than this. (But is "technically" enough?)
Odds: 1 in 3
4. Right before the last game of the season?
Why it might be: The reason Favre came back at the beginning of the season instead of waiting until Week 5 (as would have been a good idea to keep healing the various and sundry body parts that are giving him trouble) must be that he's really invested in his consecutive starts streak. If he played through the Philadelphia game on December 26th, he would get to 300. Much like Bobby Knight quitting in midseason after getting to 900 wins despite insisting numbers didn't matter, Favre might just skip the season finale at Detroit after getting to a nice, round number.
Why it might not be: Either because at that point, he might as well finish the season (although it would depend on how much Philly's D hit him) or because he will already have quit.
Odds: 1 in 20
If anybody wants to start a betting pool, I'm taking December 6th. (Although I'm worried that's too late.)