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Practically speaking, when noise is reduced to a point where additional subs provide no observable increase in SNR or more precisely; from Wikipedia...
"Signal averaging is a signal processing technique applied in the time domain, intended to increase the strength of a signal relative to noise that is obscuring it. By averaging a set of replicate measurements, the signal-to-noise ratio, S/N, will be increased, ideally in proportion to the square root of the number of measurements."
On the other hand acquiring faint signal may require many more subs. Loosely speaking...
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