I think you were thinking about too late PoDs (1917/8 or so).
A sketch of the TL for such long war would be something like this:
- Germany is able to crush France in the autumn of 1914: not full invasion, but let it temporarily inoffensive.
- Russia falls into pre-revolutionary anarchy and chaos earlier (maybe during 1915) than OTL and in a harsher mood (maybe triggered by some Rasputin-related scandal and anticipated famine in some areas), which allows Germany and A-H to perform a deeper invasion into Russian territory (West is under relative control), while the Bolsheviks start to control other areas, leaving a Russian puzzle of 'white' revolutionary areas, 'red' revolutionary areas and Central Powers' controlled areas, in a long and devastating situation of warfare.
- The earlier and bigger successes of the German/A-H in this WW TL boost more ambitious and aggressive military plans in the West (maybe invasion of NL or Denmark), which the UK can't halt.
- However, the prolonged war in the East starts to damage the internal social climate of the Central Powers by teh early 1920s: A-H finally implodes and the Habsburgs have to exile in Bavaria. Germany is too busy dealing with the political aftermath of the A-H implosion, which finally also affects German stability. France and UK are able to counter-attack.
- A Nationalist Republican revolution overthrown the Kaiser and the other German Princes by 1922-23 and take the leadership of the counter-offensive against UK and France. Russia is still taken by the chaos and civil war.
- After a couple of years of war in the West Front, the German Nationalists are able to subdue France again and the UK calls for a ceasefire, which might save France from total occupation.
- In the meantime, the resulting entities from A-H implosion have been fighting against Italy and Serbia: after France is defeated, Germany occupies Northern Italy by the late 1920s, avoiding any further conflict between them and the new Balkan states, which fall into the German orbit.
- At some point during the 1930s, the Bolsheviks are able to complete their control over non-occupied Russia and calls for a treaty of peace, as they are in too bad shape for fighting against Germany any longer.