Define "win"?
In terms of a favourable peace (France militarly neutered, the Brits out of European affairs, annexing much of western Russia?) probably up till the yanks entered the war. After all, the british financial position was increasingly precarious and the Sovs had taken a right beating in the opening phases of Barbarossa. Of cause, the problem is getting the Nazis to angle for a favourable treaty then rather than push on in hopes of taking it all...
In terms of "teh nazi ubermensh rulz teh wrold"/total conquest of Britain and the USSR... never.