How often do supernovae occur? Type Ia supernova are used as standard candles. But they also are transient events. This means that to determine the distance of a galaxy using supernovae, you have to wait for one to occur. How often do type Ia supernovae typically occur in a galaxy ?
There is a lot of scope to provide a very detailed answer here. The rate depends on what sort of a galaxy you are considering and when, what its star formation rate is (or was) and what its total stellar mass is.

A good reference is the Annual Review of Astrophysics article by Maoz et al. (2019). This says that for a Sbc galaxy like the Milky Way, the specific rate ( the rate per unit of stellar mass in the galaxy) of type Ia supernovae is $10^{-13}$ per year per solar mass of stars. For a Milky Way stellar mass of $\sim 6\times 10^{10} M_\odot$, this means a rate of 0.006 per year, or one type Ia SNe every $\sim 200$ years.

In the local universe this rate scales as about $M^{-0.5}$, where $M$ is the galaxy mass (valid over about 5 orders of magnitude for $M&gt;10^7 M_\odot$; Brown et al. 2018). So the specific rate of type Ia supernovae is larger in smaller galaxies.

The rate also decays after a galaxy is formed, or after a burst of star formation as something like $t^{-1}$.