Obviously, it depends on how you define "begun to making significant difference" and "war was already decided".

 

I'd say, the war was decided by the winter of 1941/42, once the Blitzkrieg completely lost its momentum and Eastern Front switched to the war of attrition which Axis had no hope of winning. Despite losing immense chunk of their territory, Soviets had Central Planning for over a decade and were uniquely efficient at war-time economy - while Reich's economy was a disogranized mess (a lot of centralization of economy that is attributed to NSDAP IRL was inherited from Weimar republic; Nazis weren't great at reforms) that was simply incapable of handling protracted war effort. The other Axis nations were even less prepared to deal with it.

 

Arguably, Axis had some chance if the southern offensive (operation Braunschweig; battle for Stalingrad) was successful (i.e. "war was not yet decided"). However, even this was over by the winter of 1942/43, while US started "making difference" during the summer of 1943 at the earliest (landing in Sicily) - if not in 1944 (Normandy).

In theory, battle of Kursk might be considered as the final decisive battle, but there is little to support this position. It was nothing but desperation at this point. Reich couldn't hope to win the war even if it somehow emerged victorious (in hindsight it is obvious how unlikely it was for Soviets to lose the battle), only to avoid unconditional surrender.

 

Finally, there is the question of Lend-Lease which supposedly changed the course of war. The problem here is that this theory doesn't hold water. It was not the Soviets who were the main beneficiary, but British. Soviets got only ~20% of the Lend-Lease, and their shipments truly took off only in 1943. It definitely accelerated defeat of Axis by a few months (if not a full year), but it's hard to treat LL as a war-changing factor.

In fact, one could argue that Lend-Lease was a war-changing factor in Asia (war with Japan), since it secured Soviet assistance in defeat of Kwantung army in China - a type of war US was hardly looking towards to.