The USSR never claimed to or practice "true communism". The USSR was officially Marxist which meant that they where attempting to create a transition to global revolution in which communism would then be established.

In terms of their practice of socialism the USSR had two main shifts. One occurred under Stalin who attempted to develop the idea of "socialism in one nation" after the failure of most of the western Marxist revolutions of the late 1910s and early 1920s. The other large policy shift occurred after Stalin's death and Khrushchev coming into power and his attempted reforms that shifted the USSR's economy by trying to reduce the amount of centralized collectivism/planning and attempts at trying to diversify and develop the USSR's internal economy.

Earlier than you'd think. If you believe the anarchists, almost immediately. See Emma Goldman's My Disillusionment in Russia, from 1923, where she describes her meeting with Lenin in 1920. This was followed by the Kronstadt Rebellion and Lenin's adoption of the New Economic Policy (which was called "state capitalism"). This was when it really became clear to other sectors of the international left that a major departure from socialist ideals had occurred. It's sometimes not recognised enough that the anarchist and Trotskyist left were major opponents of Soviet tyranny and authoritarian communism, having fought against it almost from the beginning.

Also see Alexander Berkman's critique of Soviet Communism in his book "ABC of Anarchism" from 1928.

Depends who you ask. Obviously the die-hard Soviets would insist that it never abandoned the pursuit of communism. Trotskyists would say it had turned off the road by the purges of the late 1920s and 1930s. Anarchists would say during the Civil War, with the confirming blow coming at Kronstadt in 1921.

I think somewhere in Society of the Spectacle, Guy Debord posits that after the death of Stalin, most of the Soviet leadership stopped sincerely believing they were following Marxist-Leninist doctrine, and it just became their justification for power and the continuation of the Soviet state.