= Solution
For independent <electron> arrivals, the shot-noise counting model gives mean transferred charge $Q=I\Delta t=10^{-15}\,\mathrm C$ and count $\bar N=Q/e\simeq6.24\times10^3$. Poisson statistics imply
$$
\boxed{\sigma_Q=e\sqrt{\bar N}=\sqrt{eI\Delta t}\simeq1.27\times10^{-17}\,\mathrm C},\qquad
\frac{\sigma_Q}{Q}\simeq1.27\%.
$$
This is about 79 <electron> charges rms. It assumes Poisson <shot noise>; charge discreteness by itself does not rule out correlations, <thermal noise> or other apparatus noise.
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