It is hard if not impossible to discern an accurate date where the average frontline soldier knew enough information in a closed totalitarian state to decide that the war was done.

But if you need an answer, the author Zaloga would likely say Summer, 1944, because only then when both Allies had clearly triumphed east and west in Normandy and Operation Bagration, did the German officer corps feel galvanized to attempt its best attempted assassination of the Third Reich's leadership during Operation Valkyrie. So at least by this time, many if not most of the officer corps felt defeat was inevitable, and reacted either with loyal fatalism, or violent rebellion.

As to the average frontline solider, I think that a similar date might be useful. Stalingrad, Kursk, or even El Alemein were clear defeats for the Germans, but only on specific operational fronts. Army Group North and Center had barely caved in, and Western Europe remained fully occupied. As late as 1943, the Wehrmacht still won clear tactical victories, made local advances, and deserted only by the thousand. But by June, 1944, the Germans were surrounded or on the retreat in almost all sectors and were deserting or surrendering by the tens of thousands.