@Alan McDougall,
Almost certainly (though it's impossible to say
how early it would have happened). Stalin deeply sabotaged his own military, his own intelligence services, his own preparations for a
patently obvious German invasion, and he destructively meddled with his own Generals' operational plans after the war had begun.
After the Germans invaded, Stalin was so shocked (having repeatedly denied the possibility) that he went into some sort of catatonic state for a couple weeks. The Red Army leadership was so paralyzed by fear of him that they couldn't even take their own initiative to respond.
Think about this figure -- during the first 7 months of the war the Nazis captured more than
3 million Red Army Soldiers (virtually all of whom died from starvation or summary execution). That's not even counting the millions that died on the battlefield. In other words, the beginning of the war was a complete catastrophe. Stalin gets credit for
one crucial maneuver during this time -- they evacuated the Soviet war industry East of the Ural Mountains before the Wermacht could capture it.
After the Battle of Moscow in Dec 1941, Pearl Harbor happened and Germany declared war on the USA. Because of this the USA began to supply the Red Army, especially with jeeps, trucks, etc. This proved to be critical material support later in the war, especially at some battles like Kursk and like Operation Bagration. Also, because Japan turned their eyes to the Pacific instead of Russia, the USSR was able to move troops away from defense against Japan and bring them to the defense of Moscow. In 1943, the pivotal Battle of Kursk was won in part because the Allies invaded Sicily, so Hitler withdrew from the battle (and Germany couldn't mount a single other offensive in the East for the rest of the war). So there were external circumstances that only happened later in the war that allowed it to swing more and more in the favor of the USSR. So the
duration of the war might not have been changed so dramatically; but Germany's penetration into the USSR certainly would have, and the complete savagery they wrought on the occupied territories would have been shortened and reduced.