1922, in the sense that if a nation exists, its eventual fall is as inevitable as your death or mine.
Sure, as in five billion years the sun will turn into a nova and annihilate all higher live on earth.
I think, many posters in this thread mix up "Soviet Union as a state", "Soviet Union as leader of its own powerbloc" and "Soviet Union as 'communist'", so there is no sure answer to give. I think that Soviet Union as a rump state could have existet until the August coup, but it's politics and economy would be very much likely to be like today's Russia.
