= Solution
The <gas impingement flux> onto a surface follows from the normal component of the <Maxwell-Boltzmann distribution>:
$$
J=\frac{P}{\sqrt{2\pi m k_BT}}.
$$
Here $P=0.1\,\mathrm{Pa}$ and $m=18(1.66\times10^{-27})=2.99\times10^{-26}\,\mathrm{kg}$, giving
$$
J\simeq3.60\times10^{21}\,\mathrm{m^{-2}s^{-1}}.
$$
Assume a conventional <monolayer> density $N_s=10^{19}\,\mathrm{m^{-2}}$ and unit <sticking coefficient>. The incident-monolayer time is
$$
t_{\mathrm{ML}}=\frac{N_s}{J}\simeq2.8\times10^{-3}\,\mathrm s.
$$
Thus \b[a few milliseconds] is the usual contamination-time estimate at this pressure.
The assumed molecular footprint matters: neither $N_s$ nor the <sticking coefficient> is specified. More generally $t_{\mathrm{ML}}=N_s/(sJ)$ if the sticking probability is $s$ and <desorption> is negligible. Water can desorb rapidly from clean <graphene> at room temperature, so the incident-monolayer estimate does not establish that a stable, complete water layer actually forms in that time. Residues and defects can increase local adsorption.
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