Answer
$RC$ has units of seconds.
Work Step by Step
Resistance R has these units because $R=\frac{V}{I}$: $$\frac{volt}{ampere}=volt\frac{second}{coulomb}.$$
Capacitance C has these units because $C=\frac{Q}{V}$:
$$\frac{coulomb}{volt}.$$
$RC$ has units of seconds.