  We estimate the evaporation timescale for spherical HI clouds consisting of
the cold neutral medium surrounded by the warm neutral medium. We focus on
clouds smaller than 1pc, which corresponds to tiny HI clouds recently
discovered by Braun & Kanekar and Stanimirovi{\'c} & Heiles. By performing
one-dimensional spherically symmetric numerical simulations of the two-phase
interstellar medium (ISM), we derive the timescales as a function of the cloud
size and of pressure of the ambient warm medium. We find that the evaporation
timescale of the clouds of 0.01 pc is about 1Myr with standard ISM pressure,
$p/k_{B}\sim 10^{3.5}$ K cm$^{-3}$, and for clouds larger than about 0.1 pc it
depends strongly on the pressure. In high pressure cases, there exists a
critical radius for clouds growing as a function of pressure, but the minimum
critical size is $\sim$ 0.03 pc for a standard environment. If tiny HI clouds
exist ubiquitously, our analysis suggests two implications: tiny HI clouds are
formed continuously with the timescale of 1Myr, or the ambient pressure around
the clouds is much higher than the standard ISM pressure. We also find that the
results agree well with those obtained by assuming quasi-steady state
evolution. The cloud-size dependence of the timescale is well explained by an
analytic approximate formula derived by Nagashima, Koyama & Inutsuka. We also
compare it with the evaporation rate given by McKee & Cowie.
