At a given moment, the state of the hypothetical quantum computer with N qubits is characterized by 2^N quantum amplitudes, which are complex continuous variables restricted by the normalization condition only. Their values cannot be arbitrary, they must be under our control. For moderate N = 1000, the number of quantum amplitudes greatly exceeds the number of particles in the Universe. Thus the answer to the question in title is: When physicists and engineers will learn to keep under control this number of continuous parameters.
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