Id say Cal bear did a good enough job in the Anglo-American Nazi War
leaving aside the plausibility of that TL, its PoD is BEFORE the end of the battle of Stalingrad.
as for my 2 cents, based entirely on German actions and not Allied mistakes
- start building the Panther line early
- pull back from Tunisia
- somehow concentrate enough forces in Sicily to defeat op. Husky without scaring off the Allies from invading in the first place; this should keep Italy in the war
- cancel Op. Zitadelle at the last possible second and then get lucky by correctly guessing the location of the main Soviet counteroffensives
- launch Op. Eisenhammer as early as possible
- get immensely lucky with the weather when Op. Overlord begins AND guess the location of the Allied landings
Honestly, I don't see how you could prolong things a whole lot further. The strategic bomber offensive was crippling Germany's entire transportation infrastructure and the oil reserves necessary for at least some kind of maneuver warfare in the east were running out anyway, which keeping a lot more tanks and trucks in the field by carefully husbanding them will only exacerbate.
I'd say the winter of '45 is about the absolute latest. Maybe spring '46 if the Norway garrison refuses to surrender for whatever reason.