if you want a houserule where you can draw, and that limits the game. here is a suggestion.
From turn 4, increase US production by 10 IPC per turn. The game ends when the axis only holds 1 VC.
If the game ends before turn 13, the allies win, if it ends between turn 13 and 17, its a draw. if the axis survives turn 17, then the axis wins.
I am not sure what the exact turnlimits and us boost in production should be, but this will guarantee that the game ends by turn 17.
Another version would be to count either VCs or production by the end of turn 12, and have some limits on how many VCs is a win, draw or loss.