Honestly, probably when the United States entered the war. Not to make the contributions of the Soviet Union seem smaller than they were, but before the United States entered the war, there was still the possibility (though clearly greatly reduced after the Battle of Britain) the United Kingdom could fall and then Germany could turn their full power against the Russians and potentially win battles they might have otherwise lost. It was the opening of a two-front war that defeated Germany.

So with the abandoning of invading Britain and the beginnings of invading Russia Germany's chances of defeat being fairly imminent went from being around 5% to about 85%. Then the United States entering the war pushed it from 85% to 100%.

The British weren't even subtle about that being the truth either. After the fall of France, Winston Churchill gave possibly the most inspirational wartime speech in history as the UK stood alone against Germany. In that speech there is a line that basically says "and we'll wait for the Americans, they have the money and people to maybe win this war".

Also, my 5% to 85% jump is a little egregious without even mild explanation. Like you said, the Germans were aiming for quick surrenders from those they invaded, that was the whole point of Blitzkrieg. With Russia it was more important than with Britain they surrender quickly. Britain can basically be invaded any time of year. Russia needs to be done while supply lines and such can be well maintained. Russia has terrible weather that has made invading even small sections near impossible for centuries, the British and French (French twice) had terrible results in attempting it. Also, Hitler diverted forces away to capture the strategically (relatively) unimportant city of Stalingrad which was a disaster. So Stalingrad was also a major turning point.

But defeat became inevitable once the United States entered the war. It was already incredibly likely though.