The best answer I've found to this question is that lend lease undeniably made the Red Army's victory faster and easier, but was not decisive enough that without it the Red Army couldn't eventually achieve the same result. Mark Harrison in this essay (Dated but still useful) puts the peak of lend lease at 13.2 billion rubbles out of 129.4 billion total.
Lend lease trucks are often brought up as being critical for sustaining the Red Army's advance, especially in major operations such as Bagration and Jassy-Kishinev. Lend lease imports were certainly enormous, but the Soviet Union had its own even larger prewar truck fleet along with captured vehicles. Lend lease trucks made up around 5% of the fleet in January 1943, 19% in January 1944, and 30% in January 1945. Very few of these trucks, or indeed any trucks (Around 12,000 out of hundreds of thousands) were used for supply operations. Their main use was in artillery penetration divisions, along with other artillery formations, as prime movers. This gave strategic artillery greater mobility, allowing it to help in quickly reducing Festungs (Areas to be defended to the death under Hitler's orders) and pockets. Corps and divisional artillery remained tractor or horse drawn. So lend lease vehicles were by no means decisive, but definitely very useful.
Source for this is a GAVTU (Main automobile directorate of the Red Army) report which summarizes vehicle usage in the Red Army.
It was enormously helpful, after Operation Barbarossa and the consequent Russian defeats, Stalin's scorched earth policy reduced Russian infrastructure greatly. In addition, the Germans advanced ferociously until the fall. That being said, Soviet output capacity from an industrial and agricultural standpoint were reduced greatly. When the Lend Lease Act was extended to Russia, food, munitions, and trucks were vital to the Russian war effort. In regards to transportation, there were two routes: firstly the Atlantic convoys through the Arctic seas and secondly through the Middle East into the Southern Caucasus region. If I'm not mistaken the Southern Caucasus route helped supply the Red Army with nearly half a million trucks which were used from Kursk until V-E Day. Additionally, Stalin himself even admitted the importance of American contribution.