You have to ask, why, exactly?
While I don't agree with SS that a "win scenario" doesn't exist, we have to keep in mind that any win scenario will have to have lower expectations than Iraq.
What does a "win scenario" look like to you? Does it matter if that scenario exists for a day/week/hour or does the "win" have to be at all persistent?
What does a Afghan "win scenario" look like to me?
1. Drive Taliban & AQ from power.
2. Kill lots of Tali & AQ.
3. Impress upon any bystanders that we will destroy, with ruthlessness & vigor, those we deem a threat...and we are indifferent to the damage caused to those between us & the threat.
4. Make it clear to the local schmoes who wind up with authority after the Taliban & AQ get the boot that if he/they don't keep their barbarous affairs from seeping outside their borders, their lives are forfeit.
5. Git while the getting's good. Do not let familiarity breed contempt.
6. No great expectations as to persistence. The swamp will have to be drained every once in a while and new generations will have to be taught not to screw with America.
We've been dick-dancing around long enough that any pull out will look like we left at their behest, not ours. Best we can do now is to kill the hell outta the current batch of savages with a surge in troops, declare victory, and go home.
Unfortunately, "win scenario" for many includes setting up a stable and not too hideous central gov't and some sort of start on building infrastructure. That is too bad, since any central gov't left to locals will degenerate and collapse in 2 years, tops, and the bricks we use to build the functional edifices of civilization will be stripped to build brick stoves and isolated little enclaves to ignorance.
Two years after we leave Astan, it will be a mess, again. Unless we are willing to smack their militants around and nation-build for the better part of a century*, it doesn't matter if we stay 6 months or 10 years, no matter, as the result is not proportional to the effort input.
The heart of the matter is the Afghans and their lack of human capital, not American resolve.
* Essentially colonize the place, civilize the locals, and build the infrastructure.