Answer
$\approx 3\times10^{14}m$
Work Step by Step
The Schwarzschild radius is given on page 963: $\frac{2GM}{c^2}$. Put in values for a typical galaxy.
In example 33-1 on page 949 the estimate is $2\times10^{41}kg$, though the text on the very same page quotes twice that value. This is a rough estimate.
$$R_{S,G}=\frac{2GM}{c^2}$$
$$R_{S,G}=\frac{2(6.67\times10^{-11}Nm^2/kg^2)(2\times10^{41}kg)}{(3.00\times10^8m/s)^2}$$
$$\approx 3\times10^{14}m $$
